core: Add validation to timeval parameters

core: Add validation to timeval parameters

Prior to this change, the timeval structures provided by users did not
go through any type of validation, therefore an invalid timeval would
result in potentially unclear or confusing errors when used later on.
Add checks to the core API functions that accept timevals and return
LIBUSB_ERROR_INVALID_PARAM if the timeval is not valid.

While at it, add some macro definitions to avoid magic numbers.

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