Add a mwnci to the tree

This adds a new util named mwnci (Milliways Nifty Commandline Interface,
or something) which can be used to list and generate random entries in
users.bb and folders.bb. This should be useful for populating test files
(e.g. when testing a migration to a new database format) and examining
broken ones. Some more work needs to be done on choosing values for more
of the user fields but it should be basically useful already. It has some
built-in help so 'cd src; make test; utils/mwnci' will get you started.

Fixes #4