SSL_CONF_FLAG: Prevent setting both CMDLINE and FILE flags

SSL_CONF_FLAG: Prevent setting both CMDLINE and FILE flags

The `SSL_CONF_CTX_set_flags` function did not prevent setting both
`SSL_CONF_FLAG_CMDLINE` and `SSL_CONF_FLAG_FILE` flags, which is an
invalid combination. This commit adds a check to prevent this and
updates the documentation to clarify that only one of these flags
can be set.

A new test case is also added to verify the correct behavior.

Fixes https://github.com/openssl/openssl/issues/15508

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
MergeDate: Tue Feb  3 09:40:04 2026
(Merged from https://github.com/openssl/openssl/pull/29752)