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/tools/networking/mosh/utempter_path.patch

@@ -0,0 +1,14 @@
+diff -ur mosh-1.3.2/src/frontend/mosh-server.cc mosh-1.3.2.patched/src/frontend/mosh-server.cc
+--- mosh-1.3.2/src/frontend/mosh-server.cc	2017-07-22 23:14:53.000000000 +0200
++++ mosh-1.3.2.patched/src/frontend/mosh-server.cc	2018-06-06 10:45:50.725352804 +0200
+@@ -351,6 +351,10 @@
+     }
+   }
+ 
++#ifdef HAVE_UTEMPTER
++    utempter_set_helper( "utempter" );
++#endif
++
+   try {
+     return run_server( desired_ip, desired_port, command_path, command_argv, colors, verbose, with_motd );
+   } catch ( const Network::NetworkException &e ) {