User keysets are saved as a complete keyset, but users could edit the file and delete event-sequence pairs or add an incomplete set.  When a keyset is used, default pairs are added for any that are missing, if the pair does not conflict with existing pairs.

When extensions were converted to features #27099, 8 new pairs were added to the 'complete' set.  All user keyset are missing these pairs.  I expect that other pairs will be added in the future.  I propose that augmented keysets should be written back to the file, with notice to the user.

Detail: check when read or when used?  All keysets are written, so fix all?