shared/fstab-util: teach fstab_filter_options() a mode where all values are returned

shared/fstab-util: teach fstab_filter_options() a mode where all values are returned

Apart from tests, the new argument isn't used anywhere, so there should be no
functional change. Note that the two arms of the big conditional are switched, so the
diff is artificially inflated. The actual code change is rather small. I dropped the
path which extracts ret_value manually, because it wasn't supporting unescaping of the
escape character properly.