internal/poll: optimize socket completion modes

internal/poll: optimize socket completion modes

FILE_SKIP_SET_EVENT_ON_HANDLE is always safe to use.
FILE_SKIP_COMPLETION_PORT_ON_SUCCESS is safe as long as the socket
is provided by an IFS provider.

While here, stop using the kindFileNet type, it doesn't provide any
value.

Fixes #77448

Change-Id: Ib3dc0d68c7ff57b6a1f15f017e60a092e4b87f46
Reviewed-on: https://go-review.googlesource.com/c/go/+/742281
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>