xinitrc and friends: Embed prompt-and-run functionality in xinitrc.

The prompt-and-run functionality used to be in xrun.  Unfortunately,
that runs programs from the context of the window manager (assuming you
started it from M5-F9 or whatever).  We can do better, and start the
program from the context of the xinitrc script, which we can change more
readily.  It seems easiest to simply scrap xrun and embed the
functionality directly into the xinitrc.

As a bonus, I've also added (and bound to M5-F10) a popup for submitting
commands to the xinitrc itself, and provided a script xinitcmd for doing
the same from the command line (though this is less useful).