core: Move parameter validation from backend to core

core: Move parameter validation from backend to core

Some functions (e.g. libusb_set_interface_alt_setting()) do not perform
sufficient parameter validation, leaving the burden on the backend to
catch invalid user input. Much of this validation is common across all
backends, yet not every backend implemented it. Fix this by moving
parameter validation to the core library functions.

This is also a good opportunity to remove the redundant
'num_configurations' field from the libusb_device structure. The value
of this field is already contained in the 'device_descriptor' member.

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