sepolicy: Clean up mls constraints.

Require equivalence for all write operations.  We were already
doing this for app_data_file as a result of restricting open
rather than read/write, so this makes the model consistent across
all objects and operations.  It also addresses the scenario where
we have mixed usage of levelFrom=all and levelFrom=user for
different apps on the same device where the dominated-by (domby)
relation may not be sufficiently restrictive.

Drop the System V IPC constraints since System V IPC is never allowed
by TE and thus these constraints are dead policy.

Change-Id: Ic06a35030c086e3978c02d501c380889af8d21e0
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>