serverutil/upspinserver: rewrite configs with deprecated Bucket field
serverutil/upspinserver: rewrite configs with deprecated Bucket field

The 'upspin setupserver' command already had code to rewrite config
files that use ServerConfig's deprecated Bucket field, but that
doesn't help users who deploy new upspinserver binaries that are
already set up. This change makes of copy that same rewriting code
in upspinserver, to migrate existing configurations in place.

Change-Id: I1f4e993aa456cde00902b0c505ebfcae68db21cb
Reviewed-on: https://upspin-review.googlesource.com/8684
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin