windows: add console codepage api

Add setter/getter for console input/output codepages. This is useful to set the console expected codepage instead of converting it to UTF-16 before writing.

Change-Id: Ie0465708e7cf68d776e23dbbeeca7bcb4db7d2bf
GitHub-Last-Rev: 93c06e2
GitHub-Pull-Request: #206
Reviewed-on: https://go-review.googlesource.com/c/sys/+/599395
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

windows/syscall_windows.go

@@ -313,6 +313,10 @@ func NewCallbackCDecl(fn interface{}) uintptr {
 //sys SetConsoleMode(console Handle, mode uint32) (err error) = kernel32.SetConsoleMode
 //sys GetConsoleScreenBufferInfo(console Handle, info *ConsoleScreenBufferInfo) (err error) = kernel32.GetConsoleScreenBufferInfo
 //sys setConsoleCursorPosition(console Handle, position uint32) (err error) = kernel32.SetConsoleCursorPosition
+//sys GetConsoleCP() (cp uint32, err error) = kernel32.GetConsoleCP
+//sys GetConsoleOutputCP() (cp uint32, err error) = kernel32.GetConsoleOutputCP
+//sys SetConsoleCP(cp uint32) (err error) = kernel32.SetConsoleCP
+//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 resizePseudoConsole(pconsole Handle, size uint32) (hr error) = kernel32.ResizePseudoConsole