memory: add xstrlcpy

Less "blow a hole in your foot" than strncpy. As also indicated by coverity.
Implementation inspired by the linux kernel (very similar to OSX's Libc
implementation as well).
coverity: fix BUFFER_SIZE_WARNING with str{n,l}cpy

Relates to issue #760

These coverity warnings are of the form:

>>>     CID 62602:  Buffer not null terminated  (BUFFER_SIZE_WARNING)
>>>     Calling strncpy with a maximum size argument of 256 bytes...

This is caused by strncpy not alway NULL-terminated the destination buffer
(for example in the case where strlen(src) >= size(dst)). It's better to
replace that with (x)strlcpy, which always NULL-terminates.

Most of these are related to the set_api_error macro, which uses strncpy.
The error struct is used (for example) in msgpack_rpc_error, where strlen is
executed on it, so it needs to be NULL-terminated. (x)strlcpy, unlike
strncpy, always NULL-terminates the destination buffer.

Relevant parts of the coverity report:

*** CID 62602:  Buffer not null terminated  (BUFFER_SIZE_WARNING)
/src/nvim/api/vim.c: 236 in vim_set_current_buffer()
230         if (try_end(err)) {
231           return;
232         }
233
234         char msg[256];
235         snprintf(msg, sizeof(msg),
              "failed to switch to buffer %d", (int)buffer);
>>>     CID 62602:  Buffer not null terminated  (BUFFER_SIZE_WARNING)
>>>     Calling strncpy with a maximum size argument of 256 bytes on
>>>     destination array "err->msg" of size 256 bytes might leave the
>>>     destination string unterminated.
236         set_api_error(msg, err);
237         return;
238       }
239
240       try_end(err);
241     }

*** CID 62603:  Buffer not null terminated  (BUFFER_SIZE_WARNING)
/src/nvim/api/private/helpers.c: 70 in try_end()
64       } else if (msg_list != NULL && *msg_list != NULL) {
65         int should_free;
66         char *msg = (char *)get_exception_string(*msg_list,
67                                                  ET_ERROR,
68                                                  NULL,
69                                                  &should_free);
>>>     CID 62603:  Buffer not null terminated  (BUFFER_SIZE_WARNING)
>>>     Calling strncpy with a maximum size argument of 256 bytes on
>>>     destination array "err->msg" of size 256 bytes might leave the
>>>     destination string unterminated.
70         strncpy(err->msg, msg, sizeof(err->msg));
71         err->set = true;
72         free_global_msglist();
73
74         if (should_free) {
75           free(msg);
/src/nvim/api/private/helpers.c: 78 in try_end()
72         free_global_msglist();
73
74         if (should_free) {
75           free(msg);
76         }
77       } else if (did_throw) {
>>>     CID 62603:  Buffer not null terminated  (BUFFER_SIZE_WARNING)
>>>     Calling strncpy with a maximum size argument of 256 bytes on
>>>     destination array "err->msg" of size 256 bytes might leave the
>>>     destination string unterminated.
78         set_api_error((char *)current_exception->value, err);
79       }
80
81       return err->set;
82     }
83

*** CID 62604:  Buffer not null terminated  (BUFFER_SIZE_WARNING)
/src/nvim/api/private/helpers.c: 592 in set_option_value_err()
586                                              opt_flags)))
587       {
588         if (try_end(err)) {
589           return;
590         }
591
>>>     CID 62604:  Buffer not null terminated  (BUFFER_SIZE_WARNING)
>>>     Calling strncpy with a maximum size argument of 256 bytes on
>>>     destination array "err->msg" of size 256 bytes might leave the
>>>     destination string unterminated.
592         set_api_error(errmsg, err);
593       }

*** CID 62605:  Buffer not null terminated  (BUFFER_SIZE_WARNING)
/src/nvim/os/server.c: 114 in server_start()
108       if (addr_len > sizeof(ip) - 1) {
109         // Maximum length of a ip address buffer is 15(eg: 255.255.255.255)
110         addr_len = sizeof(ip);
111       }
112
113       // Extract the address part
>>>     CID 62605:  Buffer not null terminated  (BUFFER_SIZE_WARNING)
>>>     Calling strncpy with a maximum size argument of 16 bytes on
>>>     destination array "ip" of size 16 bytes might leave the destination
>>>     string unterminated.
114       strncpy(ip, addr, addr_len);
115
116       int port = NEOVIM_DEFAULT_TCP_PORT;
117
118       if (*ip_end == ':') {
119         char *port_end;
/src/nvim/os/server.c: 88 in server_start()
82
83     void server_start(char *endpoint, ChannelProtocol prot)
84     {
85       char addr[ADDRESS_MAX_SIZE];
86
87       // Trim to `ADDRESS_MAX_SIZE`
>>>     CID 62605:  Buffer not null terminated  (BUFFER_SIZE_WARNING)
>>>     Calling strncpy with a maximum size argument of 256 bytes on
>>>     destination array "addr" of size 256 bytes might leave the
>>>     destination string unterminated.
88       strncpy(addr, endpoint, sizeof(addr));
89
90       // Check if the server already exists
91       if (map_has(cstr_t)(servers, addr)) {
92         EMSG2("Already listening on %s", addr);
93         return;

*** CID 62606:  Buffer not null terminated  (BUFFER_SIZE_WARNING)
/src/nvim/os/server.c: 186 in server_stop()
180     void server_stop(char *endpoint)
181     {
182       Server *server;
183       char addr[ADDRESS_MAX_SIZE];
184
185       // Trim to `ADDRESS_MAX_SIZE`
>>>     CID 62606:  Buffer not null terminated  (BUFFER_SIZE_WARNING)
>>>     Calling strncpy with a maximum size argument of 256 bytes on
>>>     destination array "addr" of size 256 bytes might leave the
>>>     destination string unterminated.
187
188       if ((server = map_get(cstr_t)(servers, addr)) == NULL) {
189         EMSG2("Not listening on %s", addr);
190         return;
191       }
Merge pull request #761 'fix a few strncpy calls by using xstrlcpy'
Dead code: Remove unused vimscript functions(from FEAT_CLIENTSERVER)
API: Refactor: Duplicate/free string arguments coming from msgpack

When receiving strings *from* msgpack, we don't need to duplicate/free since
the data only lives in the msgpack parse buffer until the end of the call.

But in order to reuse `msgpack_rpc_free_object` when sending event data(which is
sent *to* msgpack), Strings must be freed, which means they must also be
allocated separately.
API: Refactor: Close/free channels when their streams reach EOF
API: Events: Return channel id from the API discover request

This refactors msgapck_rpc_{dipatch,call} to receive the channel id as
argument. Now the discovery request returns the [id, metadata] array.
API: Events: Implement channel_send_event and vimscript wrapper

This function can be used to send arbitrary objects via the API channel back to
connected clients, identified by channel id.
Build: Add 'nonnull' attributes to msgpack_rpc functions
Build: Add api/{helpers,handle}.c to CONV_SRCS
Build: Add more files to clint-files.txt and fix errors
Build: Enable valgrind's --leak-check when testing the API
API: Bugfix: Remove memory leak from set_option_to
API: Bugfix: Remove memory leak from buffer_set_slice
API: Bugfix: Remove memory leak from buffer_set_name
API: Bugfix: Remove memory leak from buffer_get_line
Merge branch 'api-events'
coverity: add travis support

Install the coverity addon in Travis-CI. To save resources (coverity scan
has an allowance), the static analysis will only be run when pushing to a
specific branch.
readme: add coverity badge
coverity: manually add coverity.sh script

This is basically the script that the coverity addon should automatically
download and call. But (I believe) due to our build configuration that
doesn't automatically work. So I'll include the script in our tree and make
adjustments until it does work. The downside to this being that we might
have to edit the script if coverity published a new version.