unix: add Getcwd to support Getwd on all BSDs

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>

unix/syscall_openbsd.go

@@ -71,6 +71,23 @@ func Getdirentries(fd int, buf []byte, basep *uintptr) (n int, err error) {
 	return getdents(fd, buf)
 }
 
+const ImplementsGetwd = true
+
+//sys	Getcwd(buf []byte) (n int, err error) = SYS___GETCWD
+
+func Getwd() (string, error) {
+	var buf [PathMax]byte
+	_, err := Getcwd(buf[0:])
+	if err != nil {
+		return "", err
+	}
+	n := clen(buf[:])
+	if n < 1 {
+		return "", EINVAL
+	}
+	return string(buf[:n]), nil
+}
+
 // TODO
 func sendfile(outfd int, infd int, offset *int64, count int) (written int, err error) {
 	return -1, ENOSYS