Use timerfd for timeout handling

Use timerfd for timeout handling

Use a new file descriptor from the timerfd system calls to handle
timeouts. On supported systems, this means that there is less hassle
figuring out when the poll() timeout should be, since
libusb_get_next_timeout() will always return 0 and the timeout events will
be triggered as regular activity on the file descriptor set.

Add API function libusb_pollfds_handle_timeouts() to detect whether
you're on a platform with the timing headache, and flesh out the
surrounding documentation.