Add new search_symbols_multiple API

Add new search_symbols_multiple API

This patch adds a new symbol searching API based on linespec.c's parser
implementation.  This allows users to find "all* matching symbols instead
of the first found match (a la lookup_symbol).

gdb/ChangeLog:

	* linespec.c (collect_info::add_symbol): Make virtual.
	(struct symbol_searcher_collect_info): New struct.
	(symbol_searcher::find_all_symbols): New method.
	* symtab.h (class symbol_searcher): New class.