minijail0: honor readonly bind mounts
minijail0: honor readonly bind mounts

linux-kernel commit 2e4b7fcd926006531935a4c79a5e9349fe51125b introduced support
for readonly bind mounts, but you can't just supply MS_RDONLY along with
MS_BIND; you have to construct an MS_BIND mount first, then do another mount
with MS_REMOUNT | MS_RDONLY.

BUG=None
TEST=platform_Minijail0

Change-Id: I1a8e2c603589b2eddcdb7a6d87059fabe17c60ba
Signed-off-by: Elly Jones <1693f95ea06ee6d7b2276cb599c3f4f33a951d9c@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/13000
Reviewed-by: Will Drewry <72416dde44ea762d19e3a7a555e3b59ce98f8eea@chromium.org>

omegaup/minijail,omegaup/minijail,omegaup/minijail,omegaup/minijail