extend -K to accept a mount propagation type

By default, minijail will mark all mounts as private.  The -K flag
allows them to skip that step which will retain all existing mount
settings.  We now have scenarios where we want to share some mount
points, so lets extend -K to accept the propagation mode.  This lets
people use -Kslave and mark all the mount points as slaves.

Bug: chromium:813131
Test: `make check` and using -Kslave allows changes in the parent namespace
Change-Id: I571e402a383ecf60a6104f87ef97b76710a34d38
5 files changed