It would create 3 0-byte files (*.shd, *.sdt, *.sid), but more is actually
needed for a message base to be "created" (i.e. a call to smb_create()).
So, MsgBase.open() now uses smb_open_sub() rather than smb_open() to initialize
theSMB  status fields with the proper default values (based on the sysop
configuration) and calls smb_crate() if the header file is empty.
Yes, normally, SCFG creates message bases, but it shouldn't have to
(e.g. a fresh install on *nix, doesn't actually start with any files in
data/subs) and now that we have JavaScript-based message lister/readers, we
really needed this support.