A bit quirky (haha), because we cannot do this during context creation - we
really want any parsing error messages to show up in the right log file and
the log handler isn't set up during context creation. So we do it on the first
real call to the backend - path_add_device or udev_assign_seat.

Also, failure to initialize the quirks subsystem just means we continue as
normal. This shouldn't be a hard failure, it just means a lot of devices won't
work properly.

If the LIBINPUT_DATA_DIR environment variable is set, that directory is used
for the data file. Only that directory, no custom override file in that case.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>