gnu: Add 'inputs' field to <user-account>; make 'shell' a monadic value.

* gnu/system/shadow.scm (<user-account>)[inputs]: New field.
  (passwd-file): Bind the 'shell' field of each account.
* gnu/system/vm.scm (%demo-operating-system): Remove 'shell' field.
* gnu/system/dmd.scm (guix-build-accounts): Store a monadic value in
  'shell'.  Add 'inputs' field.
* gnu/system.scm (operating-system-derivation): Remove 'shell' field for
  'root' account.  Add all the 'user-account-inputs' to EXTRAS.