shiny/driver/x11driver: add support for DragonflyBSD

DragonflyBSD supports all the functionality needed, but was excluded
by build tags.

Change-Id: If7fb237ad0b652686f8e32d11fc1f721b455bb91
Reviewed-on: https://go-review.googlesource.com/36475
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
4 files changed