Allow healthd to read/write /dev/__null_.

init creates a private /dev/null instance named /dev/__null__
that is inherited by healthd.  Since it is created prior to
initial policy load, it is left in the tmpfs type.
Allow healthd to inherit and use the open fd.

Change-Id: I525fb4527766d0780457642ebcc19c0fcfd1778c
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>