Refactor: libpacemaker: rename high-level fencing API

Rename pcmk_fence_action() to pcmk_request_fencing(), and its internal
equivalent pcmkfence_action() to pcmkrequest_fencing(). The change is
backward-compatible because pcmk_fence_action() has not been exposed publicly
yet.

"Fence action" can be easily confused with libcrmservice actions, liblrmd
actions, libstonithd actions, scheduler actions, and so forth.

Also, the new name makes it clearer that the caller is requesting that the
cluster perform fencing, and not directly performing fencing.