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_dragonfly.go

@@ -110,6 +110,23 @@ func Accept4(fd, flags int) (nfd int, sa Sockaddr, err error) {
 	return
 }
 
+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
+}
+
 func Getfsstat(buf []Statfs_t, flags int) (n int, err error) {
 	var _p0 unsafe.Pointer
 	var bufsize uintptr