gen-req: Make use of unsupported option --req-cn fatal

Signed-off-by: Richard T Bonhomme <[email protected]>

easyrsa3/easyrsa

@@ -1923,7 +1923,13 @@ Run easyrsa without commands for usage and commands."
 	fi
 
 	# Enforce commonName
-	export EASYRSA_REQ_CN="$file_name_base"
+	if [ "$EASYRSA_REQ_CN" = ChangeMe ]; then
+		export EASYRSA_REQ_CN="$file_name_base"
+	else
+		user_error "\
+Option conflict:
+* '$cmd' does not support setting an external commonName"
+	fi
 
 	# Output files
 	key_out="$EASYRSA_PKI/private/${file_name_base}.key"