API: libcrmcommon: New pcmk_rc_no_transaction return code

pcmk_rc_no_transaction indicates that no active transaction was found.
Atomic transactions will soon be supported for CIB changes. This code
could theoretically be used for other types of transactions later, if
and when they exist.

Ref T185

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