dissect-image: optionally, reference dissected partition device nodes by diskseq

This is useful to make the dissection logic at boot a bit safer, as we
can reference device nodes by diskseq.

This locks down dissection a bit, since it makes it harder to swap out
the backing device between the time we dissected and validated it, until
we actually mounted it.

This is not complete though, as /bin/mount would have to verify the
diskseq after opening the diskseq symlink again.

See: https://github.com/util-linux/util-linux/issues/1786

src/shared/dissect-image.h

@@ -78,6 +78,7 @@ typedef enum DissectImageFlags {
         DISSECT_IMAGE_ADD_PARTITION_DEVICES    = 1 << 20, /* Create partition devices via BLKPG_ADD_PARTITION */
         DISSECT_IMAGE_PIN_PARTITION_DEVICES    = 1 << 21, /* Open dissected partitions and decrypted partitions and pin them by fd */
         DISSECT_IMAGE_RELAX_SYSEXT_CHECK       = 1 << 22, /* Don't insist that the extension-release file name matches the image name */
+        DISSECT_IMAGE_DISKSEQ_DEVNODE          = 1 << 23, /* Prefer /dev/disk/by-diskseq/… device nodes */
 } DissectImageFlags;
 
 struct DissectedImage {