* gud.el: Add history lists to the debugging commands, so we don't

* gud.el: Add history lists to the debugging commands, so we don't
	have to retype the filename every invocation.
	(gud-gdb-history, gud-sdb-history, gud-dbx-history,
	gud-xdb-history): New variables.
	(gdb, sdb, dbx, xdb): Use them when reading the argument string.