daemon: Simplify interface with 'guix authenticate'.

daemon: Simplify interface with 'guix authenticate'.

There's no reason at this point to mimic the calling convention of the
'openssl' command.

* nix/libstore/local-store.cc (LocalStore::exportPath): Add only "sign"
and HASH to ARGS.  Remove 'tmpDir' and 'hashFile'.
(LocalStore::importPath): Add only "verify" and SIGNATURE to
* guix/scripts/authenticate.scm (guix-authenticate): Adjust
accordingly; remove the OpenSSL-style clauses.
(read-hash-data): Remove.
(sign-with-key): Replace 'port' with 'sha256' and adjust accordingly.
(validate-signature): Export SIGNATURE to be a canonical sexp.
* tests/guix-authenticate.sh: Adjust tests accordingly.