The CustomFlags feature is a bit of a hack where we just join the flags
and store in the device metadata section as a string. This makes it
inefficient to check if just one flag exists as we have to split the
string to a temporary array each time.
Rather than adding to the hack by splitting, appending (if not exists)
then joining again, store the flags in the plugin privdata directly.
This allows us to support negating custom properties (e.g. ~hint) and
also allows quirks to append custom values without duplicating them on
each GUID match, e.g.
[USB\VID_17EF&PID_307F]
Plugin = customflag1
[USB\VID_17EF&PID_307F&HUB_0002]
Flags = customflag2
...would result in customflag1,customflag2 which is the same as you'd
get from an enumerated device flag doing the same thing.
The end year is legally and functionally redundant, and more importantly causes
cherry-pick conflicts when trying to maintain old branches. Use git for history.
