Refactor: libcrmcommon: add internal API for checking for fencing action

The naming is a little awkward -- "fencing action" has multiple meanings
depending on the context. It can refer to fencer API requests, fence device
actions, fence agent actions, or just those actions that fence a node (off and
reboot).

This new function pcmk__is_fencing_action() uses the last meaning, so it does
*not* return true for unfencing ("on" actions).