bgpd: expose sender side AS path loop detection

bgpd: expose sender side AS path loop detection

The sender side AS path loop detection code was implemented since the
import of Quagga code, however it was always disabled by a `ifdef`
guard.

Lets allow the user to decide whether or not to enable this feature on
run-time.

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>