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/libmain/shared.hh

@@ -41,6 +41,9 @@ template<class N> N getIntArg(const string & opt,
     return n;
 }
 
+/* Show the manual page for the specified program. */
+void showManPage(const string & name);
+
 /* Whether we're running setuid. */
 extern bool setuidMode;