map_meta_cap use "manage_network_users" instead of is_super_admin for edit_users

[sboisvert]

I find that for a multi-site setup, the ability to edit users is inconsistent. there is a capability called "manage_network_users", but this cap if given to "regular" admin's does not give them the power to edit users.
The current code in map_meta_cap in wp-includes/capabilities.php is:

case 'edit_users':
	// If multisite these caps are allowed only for super admins.
	if ( is_multisite() && !is_super_admin( $user_id ) )
		$caps[] = 'do_not_allow';

removing any flexibility, I would suggest having this based on the "manage_network_users" capability, giving us much more flexibility in terms of assigning power to control user administration.

Note that in terms of flexibility you can always hook into the map_meta_cap filter.