Disable ANSI emulation in certain windows shells

This disables recently added ANSI emulation feature in certain Windows
shells (like ConEmu) where ANSI output is emulated by default with builtin
functionality in the shell.

MSYS (mingw) runs in cmd.exe window and it doesn't support emulation.

Cygwin doesn't even pass terminal handles to docker.exe as far as I can
tell, stdin/stdout/stderr handles are behaving like non-TTY. Therefore not
even including that in the check.

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>

term/term_windows.go

@@ -3,6 +3,7 @@ package term
 
 import (
 	"io"
+	"os"
 
 	"github.com/docker/docker/pkg/term/winconsole"
 )
@@ -114,5 +115,23 @@ func GetFdInfo(in interface{}) (uintptr, bool) {
 }
 
 func StdStreams() (stdIn io.ReadCloser, stdOut, stdErr io.Writer) {
+	var shouldEmulateANSI bool
+	switch {
+	case os.Getenv("ConEmuANSI") == "ON":
+		// ConEmu shell, ansi emulated by default and ConEmu does an extensively
+		// good emulation.
+		shouldEmulateANSI = false
+	case os.Getenv("MSYSTEM") != "":
+		// MSYS (mingw) cannot fully emulate well and still shows escape characters
+		// mostly because it's still running on cmd.exe window.
+		shouldEmulateANSI = true
+	default:
+		shouldEmulateANSI = true
+	}
+
+	if shouldEmulateANSI {
 		return winconsole.StdStreams()
 	}
+
+	return os.Stdin, os.Stdout, os.Stderr
+}