introduce two functions, map_x_s() and map_s_x(), to map

introduce two functions, map_x_s() and map_s_x(), to map
between integers and strings using a single translation table,
and use them in a few places instead of ad-hoc routines
that duplicate the table.

On passing, note that REFER_CONFIRMED is never used, and add a
few comments.

Nothing to backport here.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@76547 65c4cc65-6c06-0410-ace0-fbb531ad65f3