Need a DebuggerInfo subclass which behaves as
follows (this is handled by a switch statement in
JPDADebugger.startDebugger):

1. Does not launch any VM.

2. Does select a free dt_socket port number. The
port number should be made available with a getter
on the DebuggerInfo class after startDebugger returns.

3. Starts listening on the IDE side, i.e. uses JDI
accept mode rather than attach mode.

4. Does not open any Output tab; only the debugger
console.

Thus it mixes characteristics of the two major
existing JPDA DebuggerInfo types.

Needed for proper impl of the Ant task requested
in issue #18708.