Allow normal user to write to /var/lib/xkb.

Change /var/lib/xkb group to video. Most users should be in that group
anyway, including the default UID 1000 which we really care about.

This makes sure Xephyr, running as user, can write server-*.xkm files to
/var/lib/xkb, so it does not conflict with files created by Chromium OS
X server in /tmp.

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At least this one is an easy fix. Fixes #533.