Refactor: libcrmcommon: Return (gchar *) from pcmk__digest_operation()
Avoid a pcmk__str_copy().
The calculate_X_digest() function in pe_digest.c continue to make a
(char *) copy for now, because it will be clearer to change struct
member types in a separate commit.
Signed-off-by: Reid Wahl <nrwahl@protonmail.com>