Add wrappers for commonly used memory allocation functions. These wrappers

add an automatically generated Asterisk log message if the allocation fails
for some reason.  Otherwise, they are functionally the same, with the
exception of ast_strdup and ast_strndup.  These functions have the added
ability to accept a NULL argument without error, which will just be ignored
without generating an error. The coding guidelines have also been updated to 
reflect all of this information.  (issue #4996)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@7952 65c4cc65-6c06-0410-ace0-fbb531ad65f3