build: add missing file to Makefile.mingw
header: introduce `uv_loop_size()`

We add a function called uv_loop_size which returns the size of
the loop. It is an additional function which complements
`uv_handle_size` and` uv_req_size` in order to provide full FFI
support.

Signed-off-by: Fedor Indutny <fedor@indutny.com>
unix: define NI_MAXHOST and NI_MAXSERV if needed

OSX only exposes them if _POSIX_C_SOURCE is not defined, for instance.