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.cc

@@ -91,6 +91,7 @@ string getArg(const string & opt,
     return *i;
 }
 
+
 /* Initialize and reorder arguments, then call the actual argument
    processor. */
 static void initAndRun(int argc, char * * argv)
@@ -275,6 +276,14 @@ static void * oomHandler(size_t requested)
 }
 
 
+void showManPage(const string & name)
+{
+    string cmd = "man " + name;
+    if (system(cmd.c_str()) != 0)
+        throw Error(format("command `%1%' failed") % cmd);
+}
+
+
 int exitCode = 0;
 char * * argvSaved = 0;