cmd/browser: use keygen package instead of 'upspin keygen'
cmd/browser: use keygen package instead of 'upspin keygen'

This removes the browser's depedency on the upspin command-line tool.

Change-Id: Iffd90f3728786171a241fef06c7e54ad52489c5b
Reviewed-on: https://upspin-review.googlesource.com/13380
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>
Reviewed-by: David Presotto <d244755566b06d4c956c05eb7fe29c9fb345f41e@gmail.com>

upspin/exp,upspin/exp