Make sure exec_type is assigned to all entrypoint types.

Some file types used as domain entrypoints were missing the
exec_type attribute.  Add it and add a neverallow rule to
keep it that way.

Change-Id: I7563f3e03940a27ae40ed4d6bb74181c26148849
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>