btrfs-progs: help: recognize --help option among other options

There's a report that handling of --help in connection with other option
is confusing in some cases:

Various sub-commands to btrfs claim --help is an unrecognized option if
there are any other options on the CLI.

Examples of --help being unrecognized.

  $ btrfs filesystem defragment -v --help
  btrfs filesystem defragment: unrecognized option '--help'
  Try 'btrfs filesystem defragment --help' for more information

  $ btrfs balance start -v --help
  $ btrfs balabtrfs balance start -v --help
  btrfs balance start: unrecognized option '--help'
  Try 'btrfs balance start --help' for more information

Alternatively, some sub-commands support --help even if there are extra
options on the CLI:

  $ btrfs filesystem df -v --help
  usage: btrfs filesystem df [options] <path>

      Show space usage information for a mount point

      -b|--raw                  raw numbers in bytes
      -h|--human-readable       human friendly numbers, base 1024 (default)
      -H                        human friendly numbers, base 1000
      --iec                     use 1024 as a base (KiB, MiB, GiB, TiB)
      --si                      use 1000 as a base (kB, MB, GB, TB)
      -k|--kbytes               show sizes in KiB, or kB with --si
      -m|--mbytes               show sizes in MiB, or MB with --si
      -g|--gbytes               show sizes in GiB, or GB with --si
      -t|--tbytes               show sizes in TiB, or TB with --si

      Global options:
      --format TYPE             where TYPE is: text

Update clean_args_no_options() to detect unrecognized options properly
and do not show full help text (which usued to be done in the past but
is confusing and not pointing to the problem).

There's also a mixup of global verbosity options and command-specific
verbosity options (now deprecated), so in some commands they are
recognized.

Also handle the --help option for commands without options so the
following works:

  $ btrfs fi df -v --help /
  btrfs filesystem df: invalid option 'v'
  Try 'btrfs filesystem df --help' for more information

  $ btrfs fi df -H --help /
  usage: btrfs filesystem df [options] <path>

      Show space usage information for a mount point

      -b|--raw                  raw numbers in byte
  ...

Issue: #889
Signed-off-by: David Sterba <dsterba@suse.com>

common/help.c

@@ -87,13 +87,14 @@ int check_argc_max(int nargs, int expected)
 /*
  * Preprocess @argv with getopt_long to reorder options and consume the "--"
  * option separator.
- * Unknown short and long options are reported, optionally the @usage is printed
- * before exit.
+ * Unknown short and long options are reported. Also consume the --help
+ * option in case it's for a command without any options.
  */
 void clean_args_no_options(const struct cmd_struct *cmd, int argc, char *argv[])
 {
 	static const struct option long_options[] = {
-		{NULL, 0, NULL, 0}
+		{ "help", no_argument, NULL, GETOPT_VAL_HELP },
+		{ NULL, 0, NULL, 0 }
 	};
 
 	while (1) {
@@ -103,9 +104,13 @@ void clean_args_no_options(const struct cmd_struct *cmd, int argc, char *argv[])
 			break;
 
 		switch (c) {
-		default:
+		case GETOPT_VAL_HELP:
 			if (cmd->usagestr)
 				usage(cmd, 1);
+			break;
+		default:
+			if (cmd->usagestr)
+				usage_unknown_option(cmd, argv);
 		}
 	}
 }