gnu: services: admin: Add tailon.

* gnu/services/admin.scm
  (<tailon-configuration>, <tailon-configuration-file>): New record types.
  (tailon-configuration-files-string, tailon-shepherd-service): New
  procedures.
  (%tailon-accounts, tailon-service-type: New variables.
* doc/guix.texi (Monitoring Services: Document the Tailon service.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add gnu/tests/admin.scm.
* gnu/tests/admin.scm: New file.