script: udp_mtu fallback script config & commands

script: udp_mtu fallback script config & commands

new config variables:
 - udp_mtu = number  - size in bytes after which fallback to
   another protocol will be attempted. Default 0 (off),
   recommended 1300.
 - udp_mtu_try_proto = TCP|TLS|SCTP|UDP - protocol to fallback to.
   Default: UDP (off).
 - force_rport = yes|no - global force_rport().

new script commands:
 - udp_mtu_try_proto(TCP|TLS|SCTP|UDP) - like udp_mtu_try_proto
   above but works on a per packet basis and not globally.

Author: Andrei Pelinescu-Onciul <andrei@iptel.org>