lib: optimize sockunion_connect()

lib: optimize sockunion_connect()

This function is only called with non-blocking sockets [1], so there's
no need to worry about setting O_NONBLOCK and unsetting it later if the
given fd was a blocking socket. This saves us 4 syscalls per connect,
which is not much but is something.

Also, remove an outdated comment about the return values of this
function. It returns a 'connect_result' enum now, whose values are
self-explanatory (connect_error, connect_success and connect_in_progress).

This also fixes a coverity scan warning where we weren't checking the
return value of the fcntl() syscall.

[1] bgp_connect() and pim_msdp_sock_connect().

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>