Make sure that the necessary keys are present before starting.

Make sure that a failed download will be properly detected and not
leave behind invalid files.

Add the complete option to complete and finalize the indexes after
a larger -m run...  I've been rebuilding the indexes a lot as I
tweek the format so this has become necessary...

This is a set of scripts to make an index of all the snapshots, and
the message-id of the email that contains the hashes...

There is also the utility itself which uses this index to both find
out what snapshots are available for d/l, and to verify any snapshot
that may have already been d/l'd.