No longer leak FDs on VT switch.

On some systems, X will leak file descriptors when you switch VTs. If the system stays up long enough and you switch VTs enough, it will eventually lock up from fd exhaustion.

To demonstrate this on affected machines:

lsof -n |grep -c /dev/dri/card0
control-alt-functionkey into another VT
control-alt-functionkey back to X
lsof -n | grep -c /dev/dri/card0

The number of fds leaked varies from system to system. I saw it on a Debian desktop with a Radeon 01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Caicos XT [Radeon HD 7470/8470 / R5 235/310 OEM] and a laptop with an Intel Corporation Core Processor Integrated Graphics Controller; another machine, with an nVidia card (though I don't remember the driver settings), doesn't show the bug.

The bug was originally patched by Chris Wilson in 2016, see #204