syscall: move bsd pipe syscall

Not all BSDs have the same pipe() syscall implementation - move the Darwin/FreeBSD specific implementation into their respective OS syscall files. This will be needed to add OpenBSD syscall support.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/4823057
diff --git a/src/pkg/syscall/syscall_darwin.go b/src/pkg/syscall/syscall_darwin.go
index d43d486..11dc8aa 100644
--- a/src/pkg/syscall/syscall_darwin.go
+++ b/src/pkg/syscall/syscall_darwin.go
@@ -60,6 +60,16 @@
 func PtraceAttach(pid int) (errno int) { return ptrace(PT_ATTACH, pid, 0, 0) }
 func PtraceDetach(pid int) (errno int) { return ptrace(PT_DETACH, pid, 0, 0) }
 
+//sysnb pipe() (r int, w int, errno int)
+
+func Pipe(p []int) (errno int) {
+	if len(p) != 2 {
+		return EINVAL
+	}
+	p[0], p[1], errno = pipe()
+	return
+}
+
 // TODO
 func Sendfile(outfd int, infd int, offset *int64, count int) (written int, errno int) {
 	return -1, ENOSYS