core: Refactor initialization and how the default context is handled

core: Refactor initialization and how the default context is handled

Highlights for this change:

 - usbi_default_context is only set if libusb_init() is called with NULL.
 - All hotplug related functionality (e.g. initialization, processing) has been
   moved to hotplug.c
 - Backends are simplified by removing initialization mutexes. Mutual exclusion
   between init()/exit() is provided by default_context_lock.
 - Make hotplug types and functions part of libusbi.h with the common usbi_
   prefixes (removes hotplug.h).

Addresses issue highlighted in #855

Closes #856

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