Command-line arguments: be better at detecting absent optional args.

Previously, "--list-fingerprint --quiet" was an error.  Now, the
handler for optional arguments to "--list-fingerprint" can tell that
"--quiet" is a flag, not an argument.

This only affects flags that take an _optional_ argument, so you can
still put your torrc file in a location starting with "-".

Closes #40223.

changes/argument_parse

@@ -0,0 +1,5 @@
+  o Minor features (command-line interface):
+    - When parsing command-line flags that take an optional argument,
+      treat the argument as absent if it would start with a '-' character.
+      Arguments in that form are not intelligible for any of our
+      optional-argument flags.  Closes ticket 40223.

src/app/config/config.c

@@ -2601,8 +2601,11 @@ config_parse_commandline(int argc, char **argv, int ignore_errors)
         parsed_cmdline_free(result);
         return NULL;
       }
-    } else if (want_arg == ARGUMENT_OPTIONAL && is_last) {
+    } else if (want_arg == ARGUMENT_OPTIONAL &&
+               /* optional arguments may never start with '-'. */
+               (is_last || argv[i+1][0] == '-')) {
       arg = tor_strdup("");
+      want_arg = ARGUMENT_NONE; // prevent skipping the next flag.
     } else {
       arg = (want_arg != ARGUMENT_NONE) ? tor_strdup(argv[i+1]) :
                                               tor_strdup("");