* dwarf2read.c (dwarf2_read_index): Only allow version 3.

	* dwarf2read.c (dwarf2_read_index): Only allow version 3.
	(write_psymbols): Add 'psyms_seen' and 'is_static' arguments.
	Only emit a given psymbol once.
	(struct signatured_type_index_data) <psyms_seen>: New field.
	(write_one_signatured_type): Update.
	(cleanup_htab): New function.
	(write_psymtabs_to_index): Update.  Create psyms_seen hash.  Bump
	version to 3.
	(save_gdb_index_command): Update index documentation.