core: Introduce libusb_set_option() API function

This new function allows more flexibility in extending the library to
support more user-configurable options. It is intended to provide a
single API that can support a wide variety of needs and eliminates the
need for new API functions to set future options.

The function is introduced with a single option (LIBUSB_OPTION_LOG_LEVEL)
that replaces the libusb_set_debug() function. Documentation relating to
libusb_set_debug() and the uses of this function in the examples and
tests have been updated accordingly.

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