services: Add 'profile-service-type'.

* gnu/services.scm (packages->profile-entry): New procedure.
  (profile-service-type): New variable.
* gnu/system.scm (operating-system-directory-base-entries): Remove
  the "profile" entry.
  (essential-services): Add a PROFILE-SERVICE-TYPE instance.
  (operating-system-profile): Rewrite in terms of 'fold-services'.
* doc/guix.texi (Service Reference): Add 'profile-service-type'.
* doc/images/service-graph.dot: Likewise.