ext_gain: show the current value (issue #171)

"ext_gain" without an argument now prints the current external gain after
the usage line, as "attenuate" and "rbw" do.

Tested on a ZS407 and a tinySA BASIC: 0.0, 12.5 after "ext_gain 12.5". +16 B.

sa_cmd.c

@@ -398,7 +398,7 @@ VNA_SHELL_FUNCTION(cmd_sweeptime)
 VNA_SHELL_FUNCTION(cmd_ext_gain)
 {
   if (argc != 1 || argv[0][0] == '?') {
-    usage_printf("ext_gain -100.0..+100.0\r\n");
+    usage_printf("ext_gain -100.0..+100.0\r\n%.1f\r\n", setting.external_gain);
     return;
   }
   float o = my_atof(argv[0]);