Add "set print symbol-loading on|off".

	Add "set print symbol-loading on|off".
	* NEWS: Document new option.
	* symfile.h (print_symbol_loading): Declare.
	* symfile.c (print_symbol_loading): New global.
	(symbol_file_add_with_addrs_or_offsets): Only print "Reading symbols
	from ..." if print_symbol_loading.
	(_initialize_symfile): Add set/show print symbol-loading.
	* solib.c (solib_read_symbols): Only print "Loaded symbols for ..."
	if print_symbol_loading.

	* doc/gdb.texinfo: Document "set print symbol-loading on|off".