Refactor: libcrmcommon: New pcmk__xpath_num_nodes()
To replace numXpathResults().
The function is named num_nodes() because that's literally what it's
getting -- the number of nodes in the node set of the XPath object.
However, we could just as well name it num_results() to advertise its
significance: the number of nodes in an XPath search result.
Signed-off-by: Reid Wahl <nrwahl@protonmail.com>