core: allow libusb_set_option on the default context before libusb_init

core: allow libusb_set_option on the default context before libusb_init

This commit updates libusb_set_option to save the options if setting them on the default
context. This ensures the options 1) can be set before libusb_init(NULL, ...), and 2)
are honored even if the default context is destroyed and re-created.

Signed-off-by: Nathan Hjelm <hjelmn@google.com>