SETTABLE-PROPERTIES

[David L. Andre <DLA at MIT-AI>]

I did not get any objections to two new pathname messages
:DEFAULT-SETTABLE-PROPERTIES and :PROPERTY-SETTABLE-P.  I will implement
them and semi-obsolete the :SETTABLE-PROPERTIES on NIL in directory
lists if I don't hear any complaints by tomorrow.

For CHAOS pathnames, properties will be maintained on
(DEFAULT-PATHNAME NIL host NIL NIL T) for this purpose.  Relevant
properties which may be sent via the directory list are:

:SETTABLE-PROPERTIES	   This is what :DEFAULT-SETTABLE-PROPERTIES
			   will return.  If neither other property is
			   present, :PROPERTY-SETTABLE-P will work by
			   MEMQ on this.
:ALL-PROPERTIES-SETTABLE   This will cause :PROPERTY-SETTABLE-P to
			   always return T.
:UNSETTABLE-PROPERTIES	   :PROPERTY-SETTABLE-P will return NIL for any
			   property on this list.

Every time a DIRECTORY-LIST is done, these properties will be updated.
If they do not exist when a :PROPERTY-SETTABLE-P message is done, a
bogus DIRECTORY-LIST will be done to set them up.

:SETTABLE-PROPERTIES of T will be obsolete; that should always be a list
of symbols for the advantage of utilities such as Change File Properties
which want to use them.  I will attempt to make Change File Properties
smart about user defined properties, though I'm not sure I know how.