Add Set, Clear, IsSet and Zero methods to manipulate an *FdSet. These
implement the same functionality as the FD_SET, FD_CLR, FD_ISSET and
FD_ZERO macros in <sys/select.h>.
Change-Id: I6b7bccb98e58ee5e719096ed5743f6edcd232840
Reviewed-on: https://go-review.googlesource.com/c/sys/+/205397
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
