Fix issue 1418 by moving check of KU_KEY_CERT_SIGN and weakening check_issued()

Fix issue 1418 by moving check of KU_KEY_CERT_SIGN and weakening check_issued()

Move check that cert signing is allowed from x509v3_cache_extensions() to
where it belongs: internal_verify(), generalize it for proxy cert signing.
Correct and simplify check_issued(), now checking self-issued (not: self-signed).
Add test case to 25-test_verify.t that demonstrates successful fix

Fixes #1418

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10587)