Adding KAUTH_DEVICE_TTY_ATTACH_<TYPE>

[Elad Efrat <elad%NetBSD.org@localhost>]

Hi,

We have several pieces of code that support attaching a tty. I would
like to move them away from KAUTH_GENERIC_ISSUSER, and propose
KAUTH_DEVICE_TTY_ATTACH_ as a replacement, with extensions such as
SLIP (net/if_sl.c), STRIP (net/if_strip.c), PPP (net/ppp_tty.c),
BTUART (dev/bluetooth/btuart.c), and BCSP (dev/bluetooth/bcsp.c).

Any objections? :)

Thanks,

-e.