Notably, this change fixes the TestTCPReadWriteAllocs test because
the errnoErr wrapper is now used, elimitating the allocation for
common errnos.

The change to Dup is caused by a CL 8095 that changed the Dup* calls
to use Syscall instead of RawSyscall.

Found while working on the new iOS builders.

Change-Id: I44ab9dcad27db190e175aa149865b33944f48674
Reviewed-on: https://go-review.googlesource.com/36061Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>