cmd/upspinfs: allow -prudent
cmd/upspinfs: allow -prudent

Let those so inclined enable extra security checks.

Change-Id: I15013e89cdac83ff8aa2ff30c3e75b192b792751
Reviewed-on: https://upspin-review.googlesource.com/10100
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>
Reviewed-by: David Presotto <d244755566b06d4c956c05eb7fe29c9fb345f41e@gmail.com>

upspin/upspin,upspin/upspin