windows: add FlushConsoleInputBuffer and GetNumberOfConsoleInputEvents

This adds syscall wrappers for FlushConsoleInputBuffer and
GetNumberOfConsoleInputEvents on Windows.

Author: aymanbagabas

windows/syscall_windows.go

@@ -321,6 +321,8 @@ func NewCallbackCDecl(fn interface{}) uintptr {
 //sys	SetConsoleOutputCP(cp uint32) (err error) = kernel32.SetConsoleOutputCP
 //sys	WriteConsole(console Handle, buf *uint16, towrite uint32, written *uint32, reserved *byte) (err error) = kernel32.WriteConsoleW
 //sys	ReadConsole(console Handle, buf *uint16, toread uint32, read *uint32, inputControl *byte) (err error) = kernel32.ReadConsoleW
+//sys	GetNumberOfConsoleInputEvents(console Handle, numevents *uint32) (err error) = kernel32.GetNumberOfConsoleInputEvents
+//sys	FlushConsoleInputBuffer(console Handle) (err error) = kernel32.FlushConsoleInputBuffer
 //sys	resizePseudoConsole(pconsole Handle, size uint32) (hr error) = kernel32.ResizePseudoConsole
 //sys	CreateToolhelp32Snapshot(flags uint32, processId uint32) (handle Handle, err error) [failretval==InvalidHandle] = kernel32.CreateToolhelp32Snapshot
 //sys	Module32First(snapshot Handle, moduleEntry *ModuleEntry32) (err error) = kernel32.Module32FirstW