When ‘--help’ is given, just run ‘man’ to show the manual page

I.e. do what git does.  I'm too lazy to keep the builtin help text up
to date :-)

Also add ‘--help’ to various commands that lacked it
(e.g. nix-collect-garbage).

scripts/nix-collect-garbage.in

@@ -16,6 +16,8 @@ for my $arg (@ARGV) {
         $removeOld = 1;
     } elsif ($arg eq "--dry-run") {
         $dryRun = 1;
+    } elsif ($arg eq "--help") {
+        exec "man nix-collect-garbage" or die;
     } else {
         push @args, $arg;
     }