Add basic BGP security features

[taspelund]

Issue is to track the addition of basic BGP security features.

1. TTL Security
2. MD5 Auth on TCP session

Item (1) allows an operator to define a minimum TTL value on received BGP packets in order for them to be considered valid. Typically this is used on point-to-point eBGP sessions w/ a value of 255 or similar, to prove that the packet isn't being spoofed or routed from a source elsewhere in the network.

Item (2) allows an operator to define an auth key to be used with the TCP MD5 Option. This ensures authentication of BGP packets will fail unless the peer has also defined the same auth key, i.e. it is an anti-spoofing measure.