Do not prompt for passwords when JDBC connection already defines one

[scgray]

If either the connection or the JDBC driver defines the "password" property, then do not prompt the user for a password when connecting.

Research topic: it would be nice to do this when the property is provided as part of the JDBC URL, but I'm not sure that the URL definitions are standardized enough to be able to reliably recognize this fact. Maybe this should also include:

• Allow a driver definition and connection definition to explicitly indicate "no user/no password"
• Allow a command line flag to jsqsh and \connect that indicates the same thing, like --no-auth