Changes

mgw: rtp-patch rfc5993hr: convert to each end's respective format

The purpose of 'rtp-patch rfc5993hr' feature is to convert GSM-HR
RTP packets between RFC 5993 format that is expected on AoIP interface
(as prescribed by 3GPP) and TS 101 318 format that is emitted and
perhaps required by some legacy IP-based BTS.  However, the original
implementation of this feature was excessively brute: when enabled,
it would unconditionally "flip" the format of each RTP packet handled
by the MGW.  The operator therefore had to be careful to enable this
feature only when those old BTS models are in use, and there was no
possibility of mixing both old and new BTS on the same OsmoBSC+OsmoMGW
setup.

Modify this feature to work by autodetecting the format emitted by
each RTP peer, and send packets toward each peer in the same format
as that peer emits.

Related: OS#6059
Change-Id: I6b446ad83c540fb8b7e1aae24b78c27010212d64