On function fp_get_next_timeout if exist fprint timeout or libusb
timeout the function return the smaller of they.
But if one of that not exist and have a smaller value the function
return a timeout that not exist.
Add timeout mechanism as an asynchronous equivalent of sleeping (uru4000
needs this).
Start implementing polling infrastructure which also accounts for pending
timeouts. We don't expose file descriptors yet, but this is a start.
