Changes

Fix IuUP RTP hdr seqnr field not incremented

Previous commit add osmo_io changed mgcp_udp_send() implementation from
"return sendto()", which is documented as:
"return the number of bytes sent. Otherwise, -1 shall be returned"

to "return mgcp_udp_send_msg()", which in turn calls
"return osmo_iofd_sendto_msgb()", and which is documented as:
"\returns 0 in case of success (takes msgb ownership), -1 on error
(doesn't take msgb ownership)."

So successful return code changed from >0 (bytes sent) to ==0,
but forgot to update mgcp_send_iuup() return code path check (and also
some related function documentation calling mgcp_udp_send()".

This commit fixes all the related aspects of that return code change.

Related: SYS#6907
Fixes: 352b967d1bee4b3bc51ecc383306bad0fc5b7fbf
Change-Id: I154e1e41cd02fd4d9b88ad98fc7c4d657246c589

iuup: Increment RTP hdr seqnr even if Tx over UDP fails

This way holes can be detected. In practice it's not much important
since it would be really strange that UDP fails for a while and then it
starts working out of the blue...

Related: SYS#6907
Change-Id: I8095f3505c859650c0b83abce405067bef745975