sq: add functionality for explicitly setting primary user id flag to `userid add`?

In a review of the userid add implementation, Neal raised the question if that command should offer functionality to change/set the primary User ID flag: sequoia!1310 (comment 1012982716)

Neal's conclusion was that this should never happen implicitly.

We should decide if (and if so, with which command) we want to allow users of sq to set the primary User ID flag.