usbi_handle_transfer_completion: Remove signal to event waiters

usbi_handle_transfer_completion() is only called by backends when
handling transfer completion. The backend can only make this call
whilst holding the events lock, therefore it is pointless to send
this signal. Any threads waiting to be signaled will be woken up
and will either find that an event handler is still active or will
try to obtain the events lock and be blocked.

Event waiters are automatically signaled in libusb_unlock_events(),
so when the backend is done handling events it will release the lock
and wake up any waiters. At this point the events lock wll be free,
which is the only time waking up waiters makes sense.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>
core: Only interrupt event handlers as necessary during libusb_open()

The current behavior of libusb_open() unconditionally interrupts
any event handlers via usbi_fd_notification(). However, not all
backends, namely Win/WinCE, make changes to the pollfd list during
a device open.

This change adds a new CAP for backends (HAS_POLLABLE_DEVICE_FD),
and libusb_open() will only call usbi_fd_notification() if the
backend declares this capability.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>
core: Be more efficient by not passing internal fds to backend

There are a number of fds that the core uses internally. Currently
any events on these fds are masked out so that the backend will
skip over them during handle_events().

This change improves upon this by simply not providing these fds
to the backend.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>