bgpd: correctly schedule select() at session startup

bgpd: correctly schedule select() at session startup

On TCP connection failure during session setup, bgp_stop() checks
whether peer->t_read is non-null to know whether or not to unschedule
select() on peer->fd before calling close() on it. Using the API exposed
by thread.c instead of bgpd's wrapper macro BGP_READ_ON() results in
this thread value never being set, which causes bgp_stop() to skip the
cancellation of select() before calling close(). Subsequent calls to
select() on that fd crash the daemon.

Use the macro instead.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>