chan_sip: Allow a sip peer to accept both AVP and AVPF calls

Adapts the behaviour of avpf to only impact the format of outgoing calls. For
inbound calls, both AVP and AVPF calls will be accepted regardless of the value
of avpf in the configuration.

(closes issue ASTERISK-22005)
Reported by: Torrey Searle
Patches:
     optional_avpf_trunk.patch uploaded by tsearle (license 5334)


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@401884 f38db490-d61c-443f-a65b-d21fe96a405b