bgpd: write NOTIFY non-blockingly

bgpd: write NOTIFY non-blockingly

switching the socket to blocking may well block the entire bgpd process
for some time if our peer is overloaded (which may well be the original
reason for the NOTIFY)

The error handling is slightly different from the previous ML discussion
on this;  buffer exhaustion isn't technically a fatal TCP error, and we
should probably proceed with FSM actions according to a sent NOTIFY
(adjusting timers) even if we didn't manage to get the NOTIFY onto the
wire.

Acked-by: Leonid Rosenboim <lrosenbo@wrs.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>