Add args to signup_finished action

[rabino]

In wp-signup.php there are 2 calls to do_action( 'signup_finished' );

1) When an user sings up.
2) When a new user sing up and a new site (in Multisite) is created.

It'd be good to have a way to diferenciare this differente scenarios. Also, it'd be good to have the registered user info passed to this action.

[SergeyBiryukov]

Related: #17948

[c3mdigital]

I'm closing this in favor of #17948 which is a complete overhaul of the registration process.