Simplify the implementation and the API for stringfields;

details and examples are in include/asterisk/stringfields.h.

Not applicable to older branches except for 1.4 which will
receive a fix for the routines that free memory pools.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@88454 65c4cc65-6c06-0410-ace0-fbb531ad65f3