This commit merges changes that were developed in the "racket7" repo.
See that repo (which is no longer modified) for a more fine-grained
change history.
The commit includes experimental support for running Racket on Chez
Scheme, but that "CS" variant is not built by default.
Provide unsafe functions for working with file descriptors and
sockets. Although more functions are potentially useful, these
reflect the one scurrently exported by the C API.
