meqtrees: Fix Unix sockets to be file-based on Mac

While the underlying C++ TimBase/Net/Socket class transparently
replaces abstract (file-less) Unix sockets that only work on Linux
with the traditional file-based ones on Darwin/Mac, the meqbrowser
application still has them hardcoded. Also clean up stale socket files
at startup when it is safe to do so to avoid crashes.
meqtrees: Provide link to add-on packages

Reinstate Timba packages since they appear in sidebar of GUI file open
dialogs, making it easier to open standard scripts from meqbrowser.
Since Cattery does not exist as a subdirectory within the main Python
path, focus on Siamese instead.