rumpuser(3): New RUMP_STDOUT, RUMP_STDERR environment variables.

If set, then when rump daemonizes, it opens the path in RUMP_STDOUT
and redirects fd 1 to that (which mostly gets the kernel console
output), and opens the path in RUMP_STDERR and redirects fd 2 to that
(no idea what this gets but it's probably good to record if it ever
gets anything).

This will allow tests that rely on rump_server daemons to stash the
output for diagnostics in case, e.g., the rump kernel crashes.

PR bin/58112

(riastradh)