Disallow bad combination of arguments in keygen grind (#17251)

* Use constant for outfile sentinel value

* Require --use-mnemonic flag when --no-outfile flag passed to keygen grind

clap-utils/src/input_parsers.rs

@@ -17,6 +17,9 @@ use {
     std::{str::FromStr, sync::Arc},
 };
 
+// Sentinel value used to indicate to write to screen instead of file
+pub const STDOUT_OUTFILE_TOKEN: &str = "-";
+
 // Return parsed values from matches at `name`
 pub fn values_of<T>(matches: &ArgMatches<'_>, name: &str) -> Option<Vec<T>>
 where