main: reject explicit parameters when decrypting

[woahwhattheheck]

Fixes #436.

scrypt dec accepted --logN, -r and -p, passed them through to
scryptdec_file_prep(), and died on that function's assertion with SIGABRT
(exit 134, core dumped).

Explicit parameters are encryption-only — scrypt.1 already states that when
decrypting, the values always come from the encryption header. So this is
documented misuse; the bug is that it aborts instead of producing a diagnostic.
The existing checks in main() enforce only the all-or-none rule among the three
parameters, so an incomplete set already gives a clean error and exit 1, while a
complete set on dec reached the assertion.

This rejects the parameters in main(), alongside the checks that are already
there. Testing params.logN alone is sufficient because the all-or-none checks
above have already run, so at that point the three are either all zero or all
non-zero.

Before:

$ scrypt dec -f --logN 10 -r 1 -p 1 --passphrase dev:stdin-once good.enc out.txt
scrypt: lib/scryptenc/scryptenc.c:779: scryptdec_file_prep:
    Assertion `(P->logN == 0) && (P->r == 0) && (P->p == 0)' failed.
Aborted (core dumped)                                        # exit 134

After:

$ scrypt dec -f --logN 10 -r 1 -p 1 --passphrase dev:stdin-once good.enc out.txt
scrypt: --logN, -r and -p cannot be used when decrypting     # exit 1

The test goes into tests/09-explicit-params.sh next to the existing
partially-specified-parameter checks, rather than a new file, to keep the change
small and avoid touching Makefile.am.

I did not change scrypt info, which also ignores explicit parameters but exits 0
rather than crashing; whether it should warn seems like a separate question.

Checks. Build and the full test suite pass on macOS, Ubuntu/clang and
Ubuntu/gcc, with 09-explicit-params... SUCCESS! on all three:
https://github.com/woahwhattheheck/scrypt/actions/runs/34066975743

About this PR. I am an LLM (Claude), operating on behalf of the account owner,
and I am available to discuss this and revise the patch in response to review.