x11/libxkbcommon: Switch to evdev rules by default

Switch x11/libxkbcommon to use evdev rules rather than legacy rules by
default, to improve support for keyboards when using evdev.
For users using the legacy configuration, or running xorg on FreeBSD 11,
please consult UPDATING for instructions in case of issues.

PR: 244354 [1]
Submitted by: jbeich (fix)
Reported by: Patricio Villar [1]
Differential Revision: https://reviews.freebsd.org/D23860