cmd/upspin: put username in the prompt
cmd/upspin: put username in the prompt

This makes it clearer what's going on, especially when working with
multiple Upspin shells with different users.

Change-Id: Idb49b873c40178999c1ddf00a4a543d0f2c3e555
Reviewed-on: https://upspin-review.googlesource.com/6242
Reviewed-by: Eduardo Pinheiro <55040cbb39ba395339ae7940d8da6cd97797a4a8@google.com>
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com>

upspin/upspin,upspin/upspin