pjsip: Cleanup opus.c and fix compilation warnings

Ignore-this: cc23edcac930fcc3f9b5f0620831b265

darcs-hash:20130820103534-56b4d-2b15da3546b595a88eeabb8ab1c6db088b8414f2
pjsip: Always put useinbandfec in SDP for opus codec

Ignore-this: 47ef43bc5992d921aade47372831983

darcs-hash:20130820124129-56b4d-23f69203ca75318ed5027af4b02147beb102f19f
pjsip: Relax codec matching when doing SDP negotiation

Ignore-this: d79cf4e61b37d0982a92edd998cddd97

Match only name and sample rate (don't consider the channel count). The intent
here is to fix interoperability between implementations of opus which don't
set the channel count, as mandated by the current draft. Initially it was:

a=rtpmap:XX opus/48000

but now its:

a=rtpmap:XX opus/48000/2

darcs-hash:20130820151902-56b4d-b78bcd9d1ba8bbc7eee6b6e5fa9111a02c1a6e8a