Refactor: libcrmcommon: New pcmk__cib_abs_xpath_for()

This new function gets the absolute XPath for a CIB element.

Unfortunately we can't use the absolute XPath all the time, because
based_callbacks.c:prepare_input() and the "Mirror" logic in
cib_file.c:cib_file_process_request() both operate on CIB objects that
are part of an F_CIB_CALLDATA element within the request XML. An
absolute XPath doesn't match against this embedded CIB. Detaching the
CIB XML from the request alters the request. This interferes with some
trace messages at least, and it may have other side effects as well.

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