Have fsdb(8) only mark a filesystem dirty when it is modified.

Until this update, the fsdb(8) command always marked a filesystem
as needing a full fsck unless it was run with the -n flag which
allowed no changes to be made.

This change tracks modifications to the filesystem. Two types of
changes are tracked. The first type of changes are those that are
not critical to the integrity of the filesystem such as changes to
owner, group, time stamps, access mode, and generation number. The
second type of changes are those that do affect the integrity of
the filesystem including zeroing inodes, changing block pointers,
directory entries, link counts, file lengths, file types, and file
flags.

When quitting having made no changes or only changes to data that
is not critical to filesystem integrity, the clean state of the
filesystem is left unchanged. But if filesystem critical data are
changed then fsdb will set the unclean flag which will require a
full fsck to be run before the filesystem can be mounted.

MFC-after: 1 week
Sponsored-by: The FreeBSD Foundation