cmd/upspin: do not require a context file for keygen
cmd/upspin: do not require a context file for keygen

Prior to this, if you don't have a context, you can't run keygen,
but it could well be that you're running keygen in order to create
enough state to have an rc file and hence a context.

The fix is to treat keygen like signup in cmd/upspin, and just
stop initialization early.

Fixes #135

Change-Id: Ifae130213b6d74a715e4ce1c07bbbcf6714fb516
Reviewed-on: https://upspin-review.googlesource.com/6960
Reviewed-by: David Presotto <516b9783fca517eecbd1d064da2d165310b19759@google.com>

upspin/upspin,upspin/upspin