os: disable the use of netpoll on regular files on *BSDs.

The kqueue based netpoller always registers file descriptors with EVFILT_READ and EVFILT_WRITE.
However only EVFILT_READ notification is supported for regular files.
On FreeBSD a regular file is always reported as ready for writing, resulting in a busy wait.
On Darwin, Dragonfly, NetBSD and OpenBSD, a regular file is reported as ready for both reading and writing only once.

Updates #19093

Change-Id: If284341f60c6c2332fb5499637d4cfa7a4e26b7b
Reviewed-on: https://go-review.googlesource.com/c/156379


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>