Refactor: libcrmcommon: New pcmk__opt_fencing enum value

Nothing uses this yet. It's not necessary for filtering the options in
the fencing_params array. However, it will be useful for telling
pcmk__daemon_metadata() which option list to use and for storing the
option list as an enum in crm_resource.

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