unix: fix Select to return number of ready fds on Darwin and *BSD

Make Select's signature on Darwin and the BSDs match the one on Linux
and return the number of ready file descriptors.

Fixes golang/go#34458

Change-Id: Ia618ce34ff754f2b731d7f913cab840d7948579c
Reviewed-on: https://go-review.googlesource.com/c/sys/+/196802
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/zerrors_freebsd_386.go

@@ -3,7 +3,7 @@
 
 // +build 386,freebsd
 
-// Created by cgo -godefs - DO NOT EDIT
+// Code generated by cmd/cgo -godefs; DO NOT EDIT.
 // cgo -godefs -- -m32 _const.go
 
 package unix
@@ -1055,6 +1055,7 @@ const (
 	NET_RT_IFLIST                  = 0x3
 	NET_RT_IFLISTL                 = 0x5
 	NET_RT_IFMALIST                = 0x4
+	NFDBITS                        = 0x20
 	NOFLSH                         = 0x80000000
 	NOKERNINFO                     = 0x2000000
 	NOTE_ATTRIB                    = 0x8