bgpd: move bgp_connect_check() to bgp_fsm.c

bgpd: move bgp_connect_check() to bgp_fsm.c

Prior to this change, after initiating a nonblocking connection to the
remote peer bgpd would call both BGP_READ_ON and BGP_WRITE_ON on the
peer's socket. This resulted in a call to select(), so that when some
event (either a connection success or failure) occurred on the socket,
one of bgp_read() or bgp_write() would run. At the beginning of each of
those functions was a hook into bgp_connect_check(), which checked the
socket status and issued the correct connection event onto the BGP FSM.

This code is better suited for bgp_fsm.c. Placing it there avoids
scheduling packet reads or writes when we don't know if the socket has
established a connection yet, and the specific functionality is a better
fit for the responsibility scope of this unit.

This change also helps isolate the responsibilities of the
packet-writing kernel thread.

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