core: Change event handling lock to traditional (non-recursive) type

core: Change event handling lock to traditional (non-recursive) type

The event handling lock was previously required to be of the recursive
type because the libusb_close() path requires the lock and may be
called by a thread that is handling events (e.g. from within a
transfer or hotplug callback). With commit 960a6e75, it is possible to
determine whether the current function is being called from an event
handling context, thus the recursive lock type is no longer necessary.

References:
* http://libusb.org/ticket/82
* 74282582cc879f091ad1d847411337bc3fa78a2b
* c775c2f43037cd235b65410583179195e25f9c4a

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
[hdegoede@redhat.com: rebase on top of current master]
Signed-off-by: Hans de Goede <hdegoede@redhat.com>