Minor optimization of PropertyFileInputHandler.

Use magic property name ant.input.properties instead of
input.properties to conform with our unofficial magic property naming
convention.

Use valid choices in prompt (DefaultInputHandler only) if we have a
MultipleChoiceInputRequest.

Port old <input> testcase over to this proposal.


git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@271557 13f79535-47bb-0310-9956-ffa450edef68