directory streams

[Richard M. Stallman <RMS at MIT-AI>]

I've changed the handling of unrecognized properties in directory
streams to be to read them, not to take them as strings.
This is more general.

If SETTABLE-FILE-PROPERTIES is given as "T", it now means
the symbol T rather than the list with one element (T).
I'd prefer that someone else deal with making
M-X Change File Properties do something useful with this.