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-env/nix-env.cc

@@ -4,7 +4,6 @@
 #include "misc.hh"
 #include "shared.hh"
 #include "eval.hh"
-#include "help.txt.hh"
 #include "get-drvs.hh"
 #include "attr-path.hh"
 #include "common-opts.hh"
@@ -66,7 +65,7 @@ typedef void (* Operation) (Globals & globals,
 
 void printHelp()
 {
-    cout << string((char *) helpText);
+    showManPage("nix-env");
 }