- move ast_strdupa from channel.h to utils.h

- move ast_strdupa from channel.h to utils.h
- attempt to log an error message if the __builtin_alloca inside of ast_strdupa
  fails.
- document the fact that it is known and intended behavior for ast_strdupa to
  cause Asterisk to crash if the alloca fails
- use __builtin_expect when checking for allocation failure in all of the
  allocation wrappers

New Janitor Project!  Anywhere that we check for a successful allocation after
a call to ast_strdupa is unnecessary and should be removed.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@8356 65c4cc65-6c06-0410-ace0-fbb531ad65f3