string field manager improvements:

string field manager improvements:
use multiple memory blocks, instead of realloc(), ensuring that field pointers will never become invalid or change
don't run vs(n)printf twice when doing a field build unless required

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@8697 65c4cc65-6c06-0410-ace0-fbb531ad65f3