Refactor: libcrmcommon: pcmkxe_match() -> pcmkxe_first_child()

For clarity. This function always returns the first child element that
matches the given criteria. The "pcmk__xe_match" name doesn't make it
clear that we're matching a child rather than the given element, and it
doesn't make it clear that we're getting the first matching child.

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