Bugs, features that may be added:
- Currently "." permitted as a HASHABLE_CH. It may be desirable to
  convert all "."s to ":"s, to avoid any filename issues. For this
  reason, the not_hashable function checks that the first character
  is not ".", since otherwise these files would be hidden or clash
  with existing files.
- Could check user@domain more meaningfully, such that @domain conforms
  to alphanumeric plus dot plus dash (the standard).
- No real consistent exit code strategy at present.

Versions:
  - "qmail-hashing21Aug00": Initial public release. This included setting
	the SetUID bit on chhashdir, which I later realised meant that if
	the machine were to be compromised, arbitrary programs could be
	executed as the hashuser. Not necessarily a major hole, but I don't
	like it, so along came 0.1, and this earlier version is withdrawn.
 - 0.1:	Replaces the earlier version, but in fact the only things that have
	changed are:
	 - the Makefile - this was setting SetUID, so I've removed that to
	   avoid the issue described above
	 - the text files - to explain how to configure this package to work
	   around the SetUID issue (i.e. run everything as hashuser, or get
	   checkpassword to do the change of UID if necessary).

