Put the details consistently with event datatypes, instead
of trying to put them all in `sync', which better reflects
the extensibility of the set of synchronizable events.
A progress evt from a close input port must be initially ready,
and the primitive `peek-bytes-avail!' checks a progress evt
before checking whether the port is closed.
These changes resolve a race in `read-bytes-evt' and related evt
constructors.
The new predicates are `progress-evt?' `thread-cell-values?',
`prefab-key?', `semaphore-peek-evt?', and `channel-put-evt?'.
These were used internally, and now they appear in contract
error messages.
I originally picked "under" as the preposition to go before
a platform name, but obviously you should build "on" a
platform, and "under" suddenly annoys me. The choice of "on"
is now codified in the documentation style guide. Meanwhile,
"Unix" insted of "X" seems more clear and consistent in the
`racket/gui' docs.
More usefully, this patch also fixes a few out-of-date
platform-specific claims.
