rtp: rename modarg destination to destination_ip

rtp: rename modarg destination to destination_ip

The module argument 'source' already has special meaning as the
pa_source, however, the argument 'destination' expects an IP address.

Prior to introducing a source IP modarg for the source IP address,
rename the 'destination' argument to 'destination_ip'. Include
compatibility support for old RTP users so they don't need to change
their module usage immediately.

(changes in v2: minor formatting fixes, s/dstip/destination_ip)

Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>