Get rid of xcddefault.[ch]

The microscopic functionality provided by that file is used
only from comments.c, and only from a single place per
function in xcddefault.c. Instead of keeping that senseless
arrangement, we move the functions inline from where they
were used earlier, giving us a nice opportunity to refactor
things slightly to get around the awkward signatures those
functions used to have.

Signed-off-by: Andreas Ericsson <ae@op5.se>