archivers: Verify retrieve/unpack destdir is a directory
archivers: Allow unpacking multiple archives at a time
archivers: Recommend gpg-agent in unpack help
rgw: Successfully restore objects not owned by the bucket owner
archivers: Document that unit is a low-level subcommand
archivers.aws: Never allocate within 2 minutes of a timeslot boundary

to avoid possible double allocations within a slot if our clock doesn't
exactly match AWS.
archivers.aws: Clarify free retrieval quota in cost report
README: Document offsite archiving