BUG/MINOR: startup: don't chroot by default when set-dumpable is set

BUG/MINOR: startup: don't chroot by default when set-dumpable is set

In 3.5-dev7, the automatic chroot by default was applied with commit
79082c7c2b ("MAJOR: startup: use chroot auto by default when available"),
but it has the undesired effect to prevent us from getting a core dump
by default, even when set-dumpable is set.

Let's make sure set-dumpable disables this mechanism and document it.
This is cleaner than entering a "chroot /" because set-dumpable is
clearly identified as a debugging option that should be left commented
out after use.

Note: maybe we'll need to add a "chroot /" to all reg-tests now, as it
is no longer possible to get a core dumped by default, or we may also
use a command-line option to enforce set-dumpable.

No backport is needed.