9vx: add devcap (for auth_chuid)

To make it possible to log into 9vx using drawterm a
device #¤ (devcap) is needed, otherwise auth_chuid()
will fail, because it cannot open #¤/capuse.

This patch adds `9/port/devcap.c' from Plan 9 to
9vx/a, with only a few adaptions (see a/devcap.ed).