This commit folds in changes to both stringfields (some enhancements to the...

This commit folds in changes to both stringfields (some enhancements to the ...field_set() macro, to optimize setting strings to empty, resetting strings to shorter contents, etc.) and to chan_iax2.c, to use stringfields in the user, peer, and pvt structs. Has been running stably on iaxtel, but while iaxtel has a large registration volume, it doesn't seem to have a high call volume. So far, it seems to reduce heap usage by over half. YMMV\! Please report any IAX bugs that might involve stringfields\!

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@39203 65c4cc65-6c06-0410-ace0-fbb531ad65f3