services: syslog: Use syslog-configuration.

* gnu/services/base.scm (<syslog-configuration>): New variable.
  (syslog-service-type): Use <syslog-configuration>.
  (syslog-service): Use <syslog-configuration>.
* gnu/tests/base.scm (%avahi-os): Use <syslog-configuration>.
* doc/guix.texi (syslog-configuration-type): Add @deftp.
  (syslog-service): Update @deffn.