Refactor: libcrmcommon: New pcmk__xpath_num_results()

To replace numXpathResults().

It's arguably more technically correct to call it
pcmk__xpath_num_nodes(), but I found myself getting confused while
looking at these function calls after making that change. So I stuck
with results.

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