Various cleanups from comments in an email from Luigi Rizzo. Thank you!

- Use a cleaner syntax for declaring the allocation macros
- Fix return value for ast_strdup/ast_strndup
- remove safe_strdup from app_macro, since ast_strup does the same thing
- fix a place in app_queue where ast_calloc+strncpy was used instead of
  ast_strdup.  If you are helping out with these conversions, please watch out
  for other places where this is done.
- add a note to the coding guidelines about the fix to app_queue


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@8065 65c4cc65-6c06-0410-ace0-fbb531ad65f3