x509_vfy.c: Improve key usage checks in internal_verify() of cert chains

x509_vfy.c: Improve key usage checks in internal_verify() of cert chains

If a presumably self-signed cert is last in chain we verify its signature
only if X509_V_FLAG_CHECK_SS_SIGNATURE is set. Upon this request we do the
signature verification, but not in case it is a (non-conforming) self-issued
CA certificate with a key usage extension that does not include keyCertSign.

Make clear when we must verify the signature of a certificate
and when we must adhere to key usage restrictions of the 'issuing' cert.
Add some comments for making internal_verify() easier to understand.
Update the documentation of X509_V_FLAG_CHECK_SS_SIGNATURE accordingly.

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/12375)