remote logging: add empty string defense to remote server wrapper

See patch 6719 and logback bug report: http://jira.qos.ch/browse/LOGBACK-959

As a workaround which allows us to update to logback 1.1.1, I have added
defensive code to the String setters which converts empty strings to nulls,
which are properly handled.

I have also changed returns to System.exit, since we require this component
to run in its own domain, and have logged exceptions which may affect the
logging system itself using System.err.

Testing: Before changes, the logback 1.1.1 libraries throw the NullPointer exception;
with the changes, the server boots correctly and an alarm can be sent.

Target: master
Patch: http://rb.dcache.org/r/6775/
Require-notes: no
Require-book: no
Acked-by: Gerd