new callback parameter expansion_notify for expand_symtabs_matching

new callback parameter expansion_notify for expand_symtabs_matching

This commit adds a new callback parameter, "expansion_notify", to the
top-level expand_symtabs_matching function and to all the vectorized
functions it defers to.  If expansion_notify is non-NULL, it will be
called every time a symbol table is expanded.

gdb/ChangeLog:

	* symfile.h (expand_symtabs_exp_notify_ftype): New typedef.
	(struct quick_symbol_functions) <expand_symtabs_matching>:
	New argument expansion_notify.  All uses updated.
	(expand_symtabs_matching): New argument expansion_notify.
	All uses updated.
	* symfile-debug.c (debug_qf_expand_symtabs_matching):
	Also print expansion notify.
	* symtab.c (expand_symtabs_matching_via_partial): Call
	expansion_notify whenever a partial symbol table is expanded.
	* dwarf2read.c (dw2_expand_symtabs_matching): Call
	expansion_notify whenever a symbol table is instantiated.