activation: Build account databases with (gnu build accounts).

* gnu/build/activation.scm (enumerate, current-users, current-groups)
(add-group, add-user, modify-user, ensure-user): Remove.
(activate-users+groups)[touch, activate-user]: Remove.
[make-home-directory]: New procedure.
Rewrite in terms of 'user+group-databases', 'write-group', etc.
* gnu/build/install.scm (directives): Remove "/root".
* gnu/system/shadow.scm (account-activation): Remove (setenv "PATH" ...)
expression, which is now unneeded.