Refactor: libcrmcommon: Rename pcmk__valid_sbd_timeout()

...to pcmk__valid_stonith_watchdog_timeout(). The new name is longer but
clearer.

Previously we had pcmkget_sbd_timeout() and pcmkvalid_sbd_timeout(),
and each referred to a different timeout (SBD_WATCHDOG_TIMEOUT vs.
PCMK_OPT_STONITH_WATCHDOG_TIMEOUT).

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