Declare --props option obsolete and issue a warning message. Rationale:

Declare --props option obsolete and issue a warning message. Rationale:
- props is easy to mix up with the --prop option (for setting properties)
- the name is unspecific and inconsistent: other option names describe
  the protocol -- the way to get the properties. How is, for example,
  --httpd less about prop(ertie)s?
- two identical options easily confuse people, as can be seen in The
  FlightGear Manual, where --telnet and --props were described differently