core: Eliminate hotplug pipe, using list and event pipe instead

core: Eliminate hotplug pipe, using list and event pipe instead

To further consolidate libusb internal events, this change removes
the hotplug pipe. Hotplug messages are now kept in a list within the
context, and the event pipe is signalled when a new hotplug message
is added. When handling events, the hotplug messages will be processed
from the list instead of from a separate pipe.

This change is greatly beneficial for the Windows/WinCE backends which
do not allow pipes to be used in the WaitFor* functions.

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