aox ... younger 2 accidentally used the wrong time unit

this makes the code match the documentation
speling
base 'younger' on delete time if searching deleted mail.

this takes care of to the presumably rather common use case "undelete
whatever has been deleted in the past day", which I just now assumed to
be what "aox undelete /users/arnt/inbox younger 1" does. it wasn't. noe
it is, as a matter of DWIM.

the previous behaviour used the original insertion time.

I've also updated the documentation on the web, but not pushed that
change.