cmd/upspin-ui: update snapshot user on specifying endpoints
cmd/upspin-ui: update snapshot user on specifying endpoints

Through upspin-ui, users sign up with no endpoints provided to the key
server. Once they specify them, upspin-ui updates the record in the key
server. This change also does this for +snapshot users, which are
created automatically by the keyserver on signup.

Fix upspin/upspin#540

Change-Id: I0cd962e0a6a67260bf3bdf4af9de7a2b6c3fbecc
Reviewed-on: https://upspin-review.googlesource.com/17640
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/augie,upspin/augie,upspin/augie