Support key loading from certificate file

Support key loading from certificate file

Support loading of key and certificate from the same file if
SSL_CONF_FLAG_REQUIRE_PRIVATE is set. This is done by remembering the
filename used for each certificate type and attempting to load a private
key from the file when SSL_CONF_CTX_finish is called.

Update docs.

Reviewed-by: Richard Levitte <levitte@openssl.org>