os: only add file descriptors which are set to non-blocking mode to the netpoller
os: only add file descriptors which are set to non-blocking mode to the netpoller

Either ones where kind == kindNonBlock or those we've successfully called syscall.SetNonblock() on.
Restore blocking behavior if we detect an error registering with the netpoller and our flow was
successful in setting the inital syscall.SetNonblock().

Update #54100

Change-Id: I08934e4107c7fb36c15a7ca23ac880490b4df235
Reviewed-on: https://go-review.googlesource.com/c/go/+/420334
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Goutnik <84879473a57351dd2460656b7be39e5de0b5b235@gmail.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Run-TryBot: Yuval Pavel Zholkover <170f3b94ed768c88b460d7f0c362c1d8ffaf0cec@gmail.com>
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
Auto-Submit: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go