* gud.el: When we send a command to the debugger via gud-call,

* gud.el: When we send a command to the debugger via gud-call,
	it's annoying to see the command and the new prompt in the
	debugger interaction buffer; nuke the command and the old prompt.
	(gud-delete-prompt-marker): New variable, with extensive documentation.
	(gud-mode): Make gud-delete-prompt-marker buffer-local, and
	initialize it.
	(gud-filter-insert): If gud-delete-prompt-marker is set, delete
	the prompt, and clear gud-delete-prompt-marker.
	(gud-call): Arrange for the last prompt printed to get deleted, by
	setting gud-delete-prompt-char.