awaitEvent: better handling of EBADFs, i.e., don't
unconditionally barf() and exit if select() reports
an EBADF. See source code comments for details, but
in short, we attempt to unblock all threads to handle
the error condition before bailing out.

If only select() would indicate which file descriptor
that was the bad one.

(There's no good reason why select() errors other
than EBADF could also be handled this way, but let's
focus on it for now..)