Syslog Appender

The syslog appender is SBCL-only for now, but once mmaul/cl-syslog becomes available via Quicklisp, a port should be easy.

Can be tested like this:
```cl
CL-USER> (log4cl:add-appender log4cl:*root-logger* (make-instance 'log4cl:syslog-appender))
CL-USER> (log:info "bla")
```