Create PTY devices instead of plain files for other device types than tty

If the .umockdev device file does not specify a pre-defined device contents,
default to creating PTY device nodes, as in most cases we want to use them with
get_dev_fd() or load_script(). Just exclude SUBSYSTEM=="usb", as libusb is
trying to read from the device which hangs if you don't put anything behind
them.

With that you can use record&replay for e. g. input or DRI control devices.

Main part of https://github.com/martinpitt/umockdev/issues/5