* options.cc: Include <cerrno> and <fstream>.
	(General_options::finalize): Parse -retain-symbols-file tag.
	* options.h: New flag.
	(General_options): New method should_retain_symbol, new
	variable symbols_to_retain.
	* symtab.cc (Symbol_table::sized_finalize_symbol): Test
	should_retain_symbol map.
	* testsuite/Makefile.am (retain_symbols_file_test): New test.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/retain_symbols_file_test.sh: New file.