Fix support for proxying devices that switch interface altsettings

[xairy]

Even though #6 added basic support for devices that switch between interface altsettings, it might not work properly in certain cases: the endpoint handling threads might fail to be joined, as they might be blocked on libusb or Raw Gadget transfers.

Interrupting Raw Gadget transfers is simple: just send any signal to the endpoint threads via pthread_kill and add a no-op handler for this signal (but perhaps a signal other than SIGINT, as USB Proxy already uses this signal internally).

~~However, interrupting the libusb transfers is a problem.~~

~~A simple solution would be use the timeout argument of the libusb sync I/O API, but unfortunately libusb has a bug that makes sync I/O ignore the timeout argument.~~

~~We need to either switch to using async libusb I/O or figure out another way to interrupt libusb transfers (without fully resetting the device, as done in #14).~~

Ah, actually libusb timeouts work as expected, it's just that receive_data zeroes out timeout.