Refactor: libcrmcommon: use "message" as request/reply XML element name

It makes no sense to use function names as XML element names. Nothing has ever
checked the element name, so it should have no effect (though in the distant
future, we might want to start checking the element name).

Since we have PCMK__XA_SUBT to distinguish requests from replies, they can
share a common element name, which especially makes sense since they share some
other attribute names and meanings as well.