Isolate untrusted app ptys from other domains.

Add a create_pty() macro that allows a domain to
create and use its own ptys, isolated from the ptys
of any other domain, and use that macro for untrusted_app.
This permits the use of a pty by apps without opening up access
to ptys created by any other domain on the system.

Change-Id: I5d96ce4d1b26073d828e13eb71c48d1e14ce7d6b
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>