introduce sip_pvt_lock() and sip_pvt_unlock() wrappers to

lock these data structures.

This improve readability, and also hides the underlying
locking mechanism so it is a lot easier to add diagnostic
code, or move the object locks somewhere else, etc.

On passing, rename the lock field in sip_pvt to pvt_lock,
also for ease of readability.




git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@45786 65c4cc65-6c06-0410-ace0-fbb531ad65f3