Myth Protocol: add optional allowFallback parameter to QUERY_SG_FILEQUERY

A value of '0' will prevent it falling back to searching every storage group
if not found in the specified one. If omitted or a value of '1' is passed then
it will fall back to search all storage groups.

format is now:
QUERY_SG_FILEQUERY <host> <storagegroup> <filename> <allowfallback (optional)>