MfP4: set/unset tracking for atime, ctime, mtime, and size fields.

MfP4: set/unset tracking for atime, ctime, mtime, and size fields.
This generalizes the existing set/unset tracking for hardlink/symlink
fields and extends it to cover non-string fields.  Eventually, this
will be further extended to cover most fields.

In particular, this is needed to correctly detect when time fields
are missing (for example, reading ustar archives doesn't set atime or
ctime) for proper time restore and is helpful when trying to determine
whether to overwrite data when restoring hardlinks.

This commit updates the tests but not the docs.