Add support for managing UFS/FFS snapshots to fsck_ffs(8).
(cherry picked from commit 460ed6106cf0854caff62e4eeba8ffcd00ab0690)
(cherry picked from commit 689a9368eb60f061e10e1924f388b99e8f6c2cb2)
(cherry picked from commit 5f7acd18583116c3036e0f6e35a6f385e7e85741)
Sponsored by: The FreeBSD Foundation