Low: based: check for writeability of cib.xml.sig alongside cib.xml

The problem is that it just defers the problem for a later moment
(in case of lacking both read+write perms, it will be rather instant:

(cib_file_verify_digest) error: Could not read on-disk digest from

/var/lib/pacemaker/cib/cib.xml.sig: Permission denied (13)

) but arguably, unability to modify that file results in the same
as with plain cib.xml (in case of unability to write, the digest
will get out of sync if not fatal-erroring earlier).