Add struct-level comparison and copying of string fields to reduce the
complexity of whole-struct comparison and copying when using string
fields. The new macros do not take into account non-stringfield data.

Review: https://reviewboard.asterisk.org/r/2308/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@381017 65c4cc65-6c06-0410-ace0-fbb531ad65f3