* gold.cc (queue_initial_tasks): Pass incremental base filename

	* gold.cc (queue_initial_tasks): Pass incremental base filename
	to Output_file::open_base_file; don't print error message.
	* incremental-dump.cc (main): Adjust call to
	Output_file::open_for_modification.
	* incremental-dump.cc (main): Likewise.
	* incremental.cc (Incremental_inputs::report_command_line):
	Ignore --incremental-base option when comparing command lines.
	Ignore parameter when given as separate argument.
	* options.h (class General_options): Add --incremental-base.
	* output.cc (Output_file::Output_file):
	(Output_file::open_base_file): Add base_name and writable parameters;
	read base file into new file; print error message here.
	(Output_file::map_no_anonymous): Add writable parameter; adjust all
	callers.
	* output.h (Output_file::open_for_modification): Rename to...
	(Output_file::open_base_file): ...this; add base_name and
	writable parameters; adjust all callers.
	(Output_file::map_no_anonymous): Add writable parameter; adjust all
	callers.
	* testsuite/Makefile.am (incremental_test_4): Test
	--incremental-base.
	* testsuite/Makefile.in: Regenerate.