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       }