Move lfs_cleanerd from /usr/libexec to /libexec. This is to allow putting
the root file system on a LFS volume.

Addresses PR bin/30407. No objections in tech-userlevel@.

Include testing of the roll-forward agent, when recreating the filesystem
partial-segment by partial-segment. Each checkpoint should pass fsck_lfs -n
without errors; the results of fsck_lfs -p on non-checkpoints should also
pass fsck_lfs -n without errors.

Change LFCNWRAP{STOP,GO} to make them more suitable for snapshotting; in
particular, the caller can now choose whether to wait for the condition
to be met, and if the caller of LFCNWRAPSTOP dies or otherwise closes
the descriptor, the filesystem is started again. Updated the ckckp
regression test to use the new semantics.

dump_lfs(8) now uses the fcntls to implement LFS-style snapshotting through
the -X flag, addressing PR#33457 albeit not using fss(4). Fixed a couple
other problems with dump_lfs that manifested themselves during testing.

Allow the user to specify a file size in the ckckp regression test, including
the value zero (filling the filesystem with directories and empty file
inodes).

Be a little more careful about what we copy into the working file when;
this takes care of another class of false positives. Add copyright
assignments.

Regression test improvements:

Move the stop for LFCNWRAPSTOP to the point at which writing at segment 0
is really about to commence, since this is what the test expects (and
incidentally what a snapshotting utility wants as well).

More correctly reconstruct the on-disk state at every checkpoint, rather
than relying on the entire state at the point of wrapping to be accurate
(that is only true the first time we wrap). Add a "make abort" target to
make rerunning the test more convenient when it has failed and we're done
analyzing the failure.

Introduce two fcntl calls that freeze the filesystem right at the point
where segment 0 is being considered for writing. This allows for automated
checkpoint vailidity scanning, and could be used (in conjunction with the
existing LFCNREWIND) for e.g. snapshot dumps as well.

Include a regression test that does such scanning.

When writing the Ifile, loop through the dirty block list three times to
make sure that the checkpoint is always consistent (the first and second
times the Ifile blocks can cross a segment boundary; not so the third time
unless the segments are very small). Discovered by using the aforementioned
regression test.