APPS/cmp: prevent HTTP client failure on -rspin option with too few filenames

APPS/cmp: prevent HTTP client failure on -rspin option with too few filenames

The logic for handling inconsistent use of -rspin etc., -port, -server,
and -use_mock_srv options proved faulty.  This is fixed here, updating and
correcting also the documentation and diagnostics of the involved options.

In particular, the case that -rspin (or -rspout. reqin, -reqout) does not
provide enough message file names was not properly described and handled.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
(Merged from https://github.com/openssl/openssl/pull/20295)