neverallow untrusted_app as a mlstrustedsubject.

Assigning mlstrustedsubject to untrusted_app would undermine
the per-user isolation model being enforced via levelFrom=user
in seapp_contexts and the mls constraints.  There is no direct
way to specify a neverallow on attribute assignment, but this
makes use of a particular property of the fork permission to
prevent ever adding mlstrustedsubject to untrusted_app.

A similar restriction for app_data_file and mlstrustedobject
is also important for the same reason, but cannot be expressed
as a neverallow.

Change-Id: I5170cadc55cc614aef0cd5f6491de8f69a4fa2a0
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>