fsx revisted

fsx (tools/regression/fsx) is very useful at finding file system bugs.
I've used it to find several myself.  But it's always had one nagging
problem: because it's neither installed in the base system, nor
available from ports, it's difficult to use in CI pipelines.  I had
some free time around New Year's, so I fixed that problem.  I could've
just made a port for it, but instead I rewrote the whole thing.  The
rewrite is 100% compatible with the original (except for the -s
option, which I consider to be a bug), to the extent that identical
seed values will produce identical command sequences.  In the future,
though, I plan to add new features that will break that compatibility.
For example, fspacectl operations.

Check it out if you've ever used the original.  I'd appreciate any
feedback, feature requests, etc.

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=268938

-Alan