serverutil/perm: do not retry Watch when it is not supported
serverutil/perm: do not retry Watch when it is not supported

This prevents a log spam where we don't even expect Watch to work.

Change-Id: Ic0a85a67c6aa7dc640d77e0ca8ace771b1b5c746
Reviewed-on: https://upspin-review.googlesource.com/9861
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin