I've added the -E option, similar to ssh-keygen's, and cgtest checks
it against the OpenSSH version to ensure they match.

When I added the fmt_version field to ppk_save_parameters, I forgot to
fill it in in the special version of that struct used by cgtest.
Without that, it defaulted to 0, triggering an assertion failure.

Thanks to Pavel and his CI for pointing out what I'd forgotten: the
automated test of cmdgen.c expects that round-tripping a PPK file to
some other format and back will regenerate the identical file. Of
course, with a randomised salt in the new-look password hash, that
isn't true any more in normal usage.

Fixed by adding an option in the existing parameters structure to
provide a salt override. That shouldn't be used anywhere except
cgtest, but in cgtest, it restores the determinism we need.

Another potential (but not guaranteed) source of difference is the
automatic time-scaling of the Argon2 parameter choice. So I've turned
that off too, while I'm at it.

I don't really know why it was still in cmdgen.c at all. There's no
reason it shouldn't live in its own source file, and keep cmdgen.c for
the actual code of the key generation program!

cmdgen.c has contained code for ages to build a test main() if you
compile with -DTEST_CMDGEN. But it's painful to do so manually, since
you've still got to link in all the same supporting objects, and also
nobody can have actually done that for a while because the stub test
code hasn't been kept up to date with changes in the internal APIs
(specifically prompt_t).

Now we have the ability to include our test programs in Recipe as [UT]
or [XT] so as to leave them out of 'make install', that seems like a
useful thing to do with cmdgen's test suite. So here's a Recipe change
that builds it as 'cgtest', plus fixes for compiler warnings and bit
rot. Pleasantly, the test suite still _passes_ after those are fixed.