add memory-pool based string field management for structures

convert chan_sip sip_pvt and sip_registry structures to use string fields
add 'const' qualifiers to a few API calls that don't modify their input strings
add an asprintf() wrapper to astmm


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@7797 65c4cc65-6c06-0410-ace0-fbb531ad65f3