Make description of future-event agree with the implementation.

... and with the description in the futures visualizer docs.
setup/dirs: adjust `get-lib-search-dirs` and use if `find-dll-dir`

Add `find-dll-dir` to the end, not the beginning, so that it doesn't
override the user's directory. Improve checking for whether the DLL
directory is already covered by the library directories.
racket/runtime-path: fix problems with 'so paths

Search for versions within a library directory, instead of searching
across library directories for a version. That way, user-specific libraries
can take precedence generally.

Fix the construction of library names with versions to match `ffi-lib`.
Specifically, put the version after the suffix on platforms other than
Mac OS X.

Put the code to resolve 'so specs in one place.
Fix link to Kevin Tew's RacketCon 2012 talk.

Thanks to Evan Donahue for pointing this out.