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/exec_path.patch

@@ -0,0 +1,25 @@
+diff -ur libutempter-1.1.6/iface.c libutempter-1.1.6.patched/iface.c
+--- libutempter-1.1.6/iface.c	2010-11-04 18:14:53.000000000 +0100
++++ libutempter-1.1.6.patched/iface.c	2018-06-06 15:09:11.417755549 +0200
+@@ -60,9 +60,9 @@
+ 		_exit(EXIT_FAILURE);
+ 	}
+ 
+-	execv(path, argv);
++	execvp(path, argv);
+ #ifdef	UTEMPTER_DEBUG
+-	fprintf(stderr, "libutempter: execv: %s\n", strerror(errno));
++	fprintf(stderr, "libutempter: execvp: %s\n", strerror(errno));
+ #endif
+ 
+ 	while (EACCES == errno)
+@@ -79,7 +79,7 @@
+ 		if (setgid(sgid))
+ 			break;
+ 
+-		(void) execv(path, argv);
++		(void) execvp(path, argv);
+ 		break;
+ 	}
+ 
+Only in libutempter-1.1.6.patched: result