Add support for system messages

This change provides support for cowboy process handling requests to
handle system messages from the `sys` module.

When receving a message of the form `{system, From, Msg}` a
middleware (or sub_protocol) should return the following to handle the
message:

`{system, From, Msg, Module, Req, State}`

The system message will be handled for the middleware (or sub_protocol)
using the `sys` module. Control is returned via the `cowboy_sys`
callbacks:

* `sys_continue(Req, State)` - called when the middleware (or
sub_protocol) should continue processing as before.
* `sys_terminate(Reason, Req, State)` - called when the middleware (or
sub_protocol) should do any cleanup and then exit with reason `Reason`.

`cowboy_loop` and `cowboy_websocket` both handle system messages,
allowing live debugging of long running requests.

If a handler does not upgrade to a sub_protocol that handles system
messages, a system message received by a cowboy request process may
be handled after the current requested has finished processing but
before the next request.

Calling `sys:get_state/1,2` on a cowboy request process will return:

`{cowboy_protocol, undefined, State}`

Or:

`{Module, Req, State}`

The `Module` will be a middleware (or sub_protocol), `Req` the current
request object and `State` the state of the current module. Note the
special case of `cowboy_protocol` where `Req` is `undefined` as there is
currently no request object because the process is between requests.
This will occur when a middleware (or sub_protocol) does not handle
system messages.

The replace fun used in `sys:replace_state/2,3` on a cowboy request
process should accept, and return, a state of the form returned by
`sys:get_state/1,2`.

Calling `sys:get_status/1,2` on a cowboy request process will yield
slightly more information, and the request object will be formatted
using `cowboy_req:to_list/1`.

Note that will all `sys` module functions will work on cowboy processes,
but calling `sys:change_code/4,5` will not change the state of a
process. Also cowboy processes do not generate any `sys` debug events,
so some other functions are of limited use.