Support shell out/err splitting and exit codes

[zach-klippenstein]

adb is getting support for differentiating stdout and stderr streams, and process exit codes.

Implementation:
https://github.com/android/platform_system_core/blob/ac50d0cc5789d8462fcd2d9f6cb225e0edf9a12f/adb/shell_service.h
https://github.com/android/platform_system_core/blob/ac50d0cc5789d8462fcd2d9f6cb225e0edf9a12f/adb/shell_service.cpp

Usage: https://github.com/android/platform_system_core/blob/ac50d0cc5789d8462fcd2d9f6cb225e0edf9a12f/adb/commandline.cpp#L291

RunCommand should still be supported for combined stdin/stdout streams, but there should be an API like exec.Cmd. The old mode is called "PTY" mode, and RunCommand should probably use that. The new mode is called "protocol" mode, and multiplexes out/err streams over a single connection with a simple protocol that sends packets with stream ID headers.

The exit code is sent as a single raw byte on the kIdExit stream.

[codeskyblue]

For now, I use this way

func (c *Device) RunCommand(cmd string, args ...string) (string, error) {
    exArgs := append(args, ";", "echo", ":$?")
    outStr, err := c.commandOutput(cmd, exArgs...)
    if err != nil {
        return outStr, err
    }
    idx := strings.LastIndexByte(outStr, ':')
    if idx == -1 {
        return outStr, fmt.Errorf("adb shell error, parse exit code failed")
    }
    exitCode, _ := strconv.Atoi(strings.TrimSpace(outStr[idx+1:]))
    if exitCode != 0 {
        err = ShellExitError{strings.Join(args, " "), exitCode}
    }
    outStr = strings.Replace(outStr[0:idx], "\r\n", "\n", -1)
    return outStr, err
}

even through a little wired