cmd/upspin: fail early on signups with suffixes
cmd/upspin: fail early on signups with suffixes

The old behavior would try to signup the canonical user, but
that could be confusing and error-prone.

Change-Id: I0893ed77a0b0a805b099df646757f23c58ed1ab8
Reviewed-on: https://upspin-review.googlesource.com/8113
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

upspin/upspin,upspin/upspin