Refactor: libcrmcommon: Rename pcmk__xe_first_child -> ..._any
This is to make an upcoming change easier to review. The plan is to
rename pcmkxe_match() to pcmkxe_first_child(), taking the same four
arguments that it takes now. This way, we can do it in stages.
Signed-off-by: Reid Wahl <nrwahl@protonmail.com>