* options.h (enum Incremental_disposition): Define.

	* options.h (enum Incremental_disposition): Define.
	(class General_options): Add new options: --incremental,
	--incremental_changed, --incremental_unchanged,
	--incremental_unknown.  Add incremental_disposition_ and
	implicit_incremental_ fields.
	(General_options::incremental_disposition): New function.
	(class Position_dependent_options): Add incremental_disposition
	option.
	(Position_dependent_options::copy_from_options): Set incremental
	dispositions.
	* options.cc (General_options::parse_incremental_changed): New
	function.
        (General_options::parse_incremental_unchanged): New function.
        (General_options::parse_incremental_unknown): New function.
        (General_options::General_options): Initialize new fields
	incremental_disposition_ and implicit_incremental_.
        (General_options::finalize): Check for uasge of --incremental-*
	without --incremental.