user/cmd/cli: Improves robustness of addkey command.
user/cmd/cli: Improves robustness of addkey command.

- Does not allow secret or private key uploads by accident.

Change-Id: Iecb69d324a7a91880266590fab5659d0b455af39
Reviewed-on: https://upspin-review.git.corp.google.com/1305
Reviewed-by: Eric Grosse <f6ae9ca1078fda92fd7c2c20411c29bc6711159e@google.com>

upspin/upspin,upspin/upspin