Add version command and diagnostic logging

Implement a new Versions class which is instantiated at startup and
reflects the versions of mudpy as well as the Python interpreter on
which it's running and versions of associated Python dependencies
plus any other importable Python packages which are found to be
present in the environment.

Include a show version command which provides a relevant summary of
this information and a selftest routine to make sure it's exercised.
Also log detailed version and diagnostic information at service
start, for ease of troubleshooting and defect reporting.