cmd/upspin: prepare 'countersign' for EEIntegrityPack
cmd/upspin: prepare 'countersign' for EEIntegrityPack

Tidy up the code a little and lay out where to call Countersign
once it's implemented for EEIntegrityPack.

Update #360.

Change-Id: Ie7cec3d5a16e7ff5b111f3f04cd313878425279d
Reviewed-on: https://upspin-review.googlesource.com/9100
Reviewed-by: Eric Grosse <3706282e8b8557d7b3733fe2b8fbbcffca452208@gmail.com>

upspin/upspin,upspin/upspin