libusbX.c: addvar() to let users customize USB config/control/interface descriptor numbers to use with their device

[jimklimov]

For clarification, the addvar() idea (raised in the mailing list) regards the way for end-users to set a specific number value via configuration variables their ups.conf sections, if for some reason they don't want to default to 0 but to something else.

Ahh, gotcha! IMO, it's really only useful to have this one be a config var, if the others are too. So that would be

usb_config_index
hid_rep_index
hid_desc_index
hid_ep_in
hid_ep_out

Such user-friendly integration can be addressed separately (more so if this bites someone); so far it is great to have separate (potentially-)manageable values with recognized semantic difference :)

Cool, I'll leave this PR without that then. Doing the above set of value might let someone handle odd hardware without a code change. I think lsusb can be made to output enough information for a user to set those values in a config.

Originally posted by @kellybyrd in #2148 (comment)

[jimklimov]

PR #2331 should take care of the ability to assign the five numbers from usb.conf and suppress drivers' attempts to assign their own defaults where they deem fit. Further work in this area is still needed:

• Discovery of suitable values (nut-scanner, maybe drivers themselves in nut_libusb_open() implementations) e.g. first shot by looking at amount of descriptors and ignoring trivially small ones
• Revise driver and NUT USB library codebase which may hard-code zeroes, ones, 0x81 and 0x82 etc. to ensure that fields from usb_subdriver structure are in fact used consistently.