re-implement locking of user/1 direct editing

The attached patch prevents editing email & password fields on user/1/edit for everybody except user/1.

"Administer Users" is a very useful permission, but unfortunately provides users with an opportunity to become the superuser. I would like to re-implement the D5 feature of paranoia.

It is recognised that contrib modules regularly implement extra php permissions. Becoming user/1 gives the user the ability to execute php via these modules.