Leverage signup_id when working with $wpdb->signups

[nacin]

Followup to #15004 — we should leverage the new primary key where possible, when working with $wpdb->signups.

[jeremyfelt]

Punting to a future release. We should revisit soon after the new primary key hits in 3.7.

[nacin]

In IRC:

• seems like rewriting wpmu_validate_user_signup() would be the big one
• user-edit also treats user_login as the fake PK
• we can go without that for now

Having the primary key is better than not having the primary key, even if we aren't yet using it in core. Maybe josephscott is interested in helping implement it. :-)