The console check waits for dwl's input devices before it asks for Alt+p

[DevomB]

The compositor proof's run failed one guest check, vt-desktop-takes-keys: "menu windows: 1 before Alt+p, 1 after".

After Ctrl+Alt+F1, seatd enables dwl's seat again. dwl answers clients at once, which is what vt-desktop-back waits for. libinput reopens the input devices through the seat a moment later. The guest printed GT KEY-MENU-AFTER-VT as soon as dwl answered, and the host pressed Alt+p once, so a press that arrived before the devices were back was lost.

The check now:

• counts the /dev/input/event descriptors dwl holds before the switch;
• waits up to 15 s after the switch back until dwl holds as many again, then asks for the key;
• says so in an INFO line if they do not come back;
• names both counts in its failure message if no menu opens.

A desktop that loses its keyboard for good after a console switch still fails the check, now with the reason in its message.

Base: #585's branch, since the check's surroundings are its. It merges into #585's head for the re-proof, and into #600.