Add IP_MINTTL socket option - used to set the minimum acceptable TTL a

Add IP_MINTTL socket option - used to set the minimum acceptable TTL a
packet must have when received on a socket.  All packets with a lower TTL
are silently dropped.  Works on already connected/connecting and listening
sockets for RAW/UDP/TCP.

Add IP_RECVTTL socket option support - When set, userland receives the
incoming packet's TTL as ancillary data with recvmsg(2) call.

Allows the implementation of security mechanisms described in RFC3682 (GTSM).

Obtained-from: FreeBSD.
Submitted-by: Hasso Tepper <hasso@estpak.ee>