session: Allow multiple poll FDs per event source

Turns out that having one event source per libusb poll FD is
a bad idea. There is only a single callback for all poll FDs,
and libusb expects to be called only once per poll iteration,
no matter how many FDs triggered.

Also, they should all share the same timeout, which should get
reset on events from any polled FD. The new timeout handling made
this problem apparent, as it caused the callback to be invoked
multiple times on timeouts, once for each separate event source.

In order to fix this, change the implementation to allow for an
arbitrary number of poll FDs per event source. This number is
zero for timer FDs, one for normal I/O sources, and one or more
for libusb sources (Unix only).

Also, on Windows, do not get an additional timeout from libusb
in the event loop. This is only appropriate when polling the
libusb FDs directly, which we aren't doing on Windows.