Log Message:
pkgtools/mksandbox - update to version 1.11

+ Don't allow mounting of already mounted sandboxes, or unmounting of
unmounted sandboxes.

+ Also, add an extra "ismounted" case label to be able to tell if a
sandbox is mounted. A df(1) invocation should continue to work just
as well :)

No objections on tech-pkg@

Addresses the first part of PR pkg/51992 from Paul Goyette