bgpd: support TTL-security with iBGP

bgpd: support TTL-security with iBGP

Traditionally, ttl-security feature has been associated with EBGP
sessions as those identify directly connected external peers. The
GTSM RFC (rfc 5082) does not make any restrictions on type of
peering. In fact, it is beneficial to support ttl-security for both
EBGP and IBGP sessions. Specifically, in data centers, there are
directly connected IBGP peerings that will benefit from the protection
ttl-security provides.

Signed-off-by: Dinesh G Dutt <ddutt@cumulusnetworks.com>
Reviewed-by: Pradosh Mohapatra <pmohapat@cumulusnetworks.com>
[DL: function refactoring split out into previous 2 patches.  changes:
     - bgp_set_socket_ttl(): ret type int -> void
     - is_ebgp_multihop_configured(): stripped peer == NULL check
     - comments/whitespace]
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>