syscalls: annotate Sendfile() for race detector
Fixes #4306.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/6816054
diff --git a/src/pkg/syscall/syscall_darwin.go b/src/pkg/syscall/syscall_darwin.go
index 273a62e..7fe406a 100644
--- a/src/pkg/syscall/syscall_darwin.go
+++ b/src/pkg/syscall/syscall_darwin.go
@@ -97,7 +97,7 @@
 }
 
 // TODO
-func Sendfile(outfd int, infd int, offset *int64, count int) (written int, err error) {
+func sendfile(outfd int, infd int, offset *int64, count int) (written int, err error) {
 	return -1, ENOSYS
 }
 
@@ -334,7 +334,7 @@
 // __pthread_canceled
 // __semwait_signal
 // Proc_info
-// Sendfile
+// sendfile
 // Stat64_extended
 // Lstat64_extended
 // Fstat64_extended