Enable -Wwrite-strings

Also, fix leak from ztest_global_vars_to_zdb_args()

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13348

cmd/raidz_test/raidz_test.c

@@ -69,7 +69,7 @@ static void sig_handler(int signo)
 
 static void print_opts(raidz_test_opts_t *opts, boolean_t force)
 {
-	char *verbose;
+	const char *verbose;
 	switch (opts->rto_v) {
 		case D_ALL:
 			verbose = "no";