Refactor: libcrmcommon: Further simplify replace_xml_child()

More readability improvements. Improve variable names, limit scope of
right_val/child_val, minimize setting and unsetting of match boolean,
add comments, add tracing on matches.

Drop the "Cannot delete the search root" debug message because it's not
terribly helpful and I'd rather avoid trying to match the search root in
the first place.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>