All BSDs provide the SYS___GETCWD syscall which can be used to implement
Getwd.

Also add a test based on TestChdirAndGetwd from os/os_test.go

Change-Id: I243eae3e02a40e92afad317eb1f8a28b6032c131
Reviewed-on: https://go-review.googlesource.com/83755
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>