Disable RTS/CTS handshake by default

Fix #3

serial_darwin.go

@@ -46,6 +46,11 @@ var databitsMap = map[int]int{
 const tcCMSPAR int = 0 // may be CMSPAR or PAREXT
 const tcIUCLC int = 0
 
+const tcCCTS_OFLOW int = 0x00010000
+const tcCRTS_IFLOW int = 0x00020000
+
+const tcCRTSCTS int = (tcCCTS_OFLOW | tcCRTS_IFLOW)
+
 // syscall wrappers
 
 //sys ioctl(fd int, req uint64, data uintptr) (err error)