[Crash-utility] Hack to get filename completion for the mod command

The mod command won't do filename completion. Sometimes I set up a big ugly directory path for mod -S by starting with the dir command, using its filename completion to get the modules directory, and then command-line editing the dir command into a mod -S command before executing it. Ahem. This hacky patch allows crash to tell gdb that "mod" is a command that should get filename completion. It's pretty un-smart filename completion, e.g. doesn't know that sometimes it should be selecting from a list of modules instead of filenames, etc. And it messes with the separation of crash and gdb. But mod -S <tab><tab> seems to work. It does change the behavior of "crash> gdb help mod" and "crash> gdb mod". Bob Montgomery