Refactor: libcrmcommon: New pcmk__is_true() function

Make it inline. There's no real benefit from unit-testing this, since
it's a direct pass-through to pcmk__parse_bool() that treats NULL and
invalid as false.

Although I guess we COULD, just to make sure an invalid value that
produces pcmk_rc_bad_input returns false here.

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