Added buffer flush methods

+ serial.Port.ResetInputBuffer()
+ serial.Port.ResetOutputBuffer()

zsyscall_windows.go

@@ -49,6 +49,7 @@ var (
 	procCreateEventW        = modkernel32.NewProc("CreateEventW")
 	procResetEvent          = modkernel32.NewProc("ResetEvent")
 	procGetOverlappedResult = modkernel32.NewProc("GetOverlappedResult")
+	procPurgeComm           = modkernel32.NewProc("PurgeComm")
 )
 
 func regEnumValue(key syscall.Handle, index uint32, name *uint16, nameLen *uint32, reserved *uint32, class *uint16, value *uint16, valueLen *uint32) (regerrno error) {
@@ -161,3 +162,15 @@ func getOverlappedResult(handle syscall.Handle, overlapEvent *syscall.Overlapped
 	}
 	return
 }
+
+func purgeComm(handle syscall.Handle, flags uint32) (err error) {
+	r1, _, e1 := syscall.Syscall(procPurgeComm.Addr(), 2, uintptr(handle), uintptr(flags), 0)
+	if r1 == 0 {
+		if e1 != 0 {
+			err = errnoErr(e1)
+		} else {
+			err = syscall.EINVAL
+		}
+	}
+	return
+}