Restore the user-to-user file transfer feature

I forget who it was that said they were still using this feature in v3.18, but here you go, it's working again (the /D and /U commands). I'm not migrating any file sender/recipient info from v3.18, so only files added after upgrading to this will be downloadable from the "user" directory (if you have one).

Something that I never implemented before but noticed is missing is the removal (or dereferencing) of user-to-user files that were sent from/to a user that is then deleted. So that's still a TODO item.