Correcly handle documentation in subdirectories

* Allow empty FileHandle/SocketHandle etc instances
* Make ServerSocketHandle::listen depend on AddressingPolicy
* Replace all occurences of inet_ntoa with inet_ntop (thread safety)
* Fix handling of exceptions in ReadHelper/WriteHelper
* Fix Scheduler ERR/HUP semantics

Ipmlemented more compatible, standards conformant and flexible ClientSocketHandle::read() and readfrom() members

More layout fixes using XSLT

Extended the Sniffer example to optionally use the Scheduler
Some additional small documentation fixes

Unified emacs Local Variables

Complete TCPv6 unit testing

Set fixed column width in member docs

Add HTML tidy postprocessing and error filtering to SENFScons doxygen method
Add doclib/filter.pl INPUT_FILTER to fix example code indentation in the documentation
Add a glossary to the Socket documentation
Furthher Socket library docs (Protocol classes)

Further Socket library documentation