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).

src/nix-store/nix-store.cc

@@ -6,7 +6,6 @@
 #include "xmlgraph.hh"
 #include "local-store.hh"
 #include "util.hh"
-#include "help.txt.hh"
 
 #include <iostream>
 #include <algorithm>
@@ -29,7 +28,7 @@ typedef void (* Operation) (Strings opFlags, Strings opArgs);
 
 void printHelp()
 {
-    cout << string((char *) helpText);
+    showManPage("nix-store");
 }