Add meshlink.h, containing only the public API.

Add meshlink.h, containing only the public API.

This header file contains only the public MeshLink API, documented using
Doxygen. All functions and types are prefixed with meshlink_, and almost all of
the functions now take a meshlink_handle_t * argument, so that one can have
multiple instances of MeshLink running simultaneously, and there will be no
global variables.