This extends the socket infrastructure in libosmocore to allow
to create non-blocking sockets.
Basically, it replaces the connect0_bind1 parameter by one
flags parameter.
osmo_sock_init_ofd() is a wrapper around osmo_sock_init() which will
take care of initializing and registering a 'struct osmo_fd' for the
newly-created socket.
* use write_queue where applicable
* provide functions that work on raw FD and those with osmo_fd
* add support for multiple gsmtap instances (no global variables)