netfilter: nf_conntrack_sip: add sip_external_media logic

When enabled, the sip_external_media logic will leave SDP
payload untouched when it detects that interface towards INVITEd
party is the same with the one towards media endpoint.

The typical scenario for this logic is when a LAN SIP agent has more
than one IP address (uses a different address for media streams than
the one used on signalling stream) and it also forwards calls to a
voice mailbox located on the WAN side. In such case sip_direct_media
must be disabled (so normal calls could be handled by the SIP
helper), but media streams that are not traversing this router must
also be excluded from address translation (e.g. call forwards).

Signed-off-by: Alin Nastac <alin.nastac@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>