Allows security modules to enforce what property contents can be set by
clients.  Uses the new DixPostAccess bit to distinguish between the
existing call made during the lookup (with the old property data) and
this new call.  Note that this only applies to writes, prepends, or
appends to existing properties; for new properties the existing
DixCreateAccess hook call may be used since it includes the new data.

Refer to the XACE-Spec document in xorg-docs, section "Property Access."

Signed-off-by: Eamon Walsh <ewalsh@tycho.nsa.gov>