nixos/programs.mosh: refactor

Adds programs.mosh.withUtempter (default: true).
The option enables -with-utempter for mosh, allowing it to write to
/var/run/utmp and thus making connected sessions appear in the output
of `who -a`.

For that, a guid-wrapper is required. Also, the path to the `utempter` was
hardcoded in the resulting binary until now (so it could never been found),
thus, libutempter was patched accordingly to point to
/run/wrappers/bin/utempter which at least works when the wrapper is
configured.

pkgs/development/libraries/libutempter/default.nix

@@ -13,11 +13,13 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ glib ];
 
+  patches = [ ./exec_path.patch ];
+
   prePatch = ''
     substituteInPlace Makefile --replace 2711 0711
   '';
 
-  installFlags = [
+  makeFlags = [
     "libdir=\${out}/lib"
     "libexecdir=\${out}/lib"
     "includedir=\${out}/include"
@@ -26,6 +28,10 @@ stdenv.mkDerivation rec {
 
   meta = {
     description = "Interface for terminal emulators such as screen and xterm to record user sessions to utmp and wtmp files";
+    longDescription = ''
+      The bundled utempter binary must be able to run as a user belonging to group utmp.
+      On NixOS systems, this can be achieved by creating a setguid wrapper.
+    '';
     license = licenses.lgpl21Plus;
     platforms = platforms.linux;
     maintainers = [ maintainers.msteen ];