* Fix /proc not being mounted read-only.
* Fix other read-only directories not actually being mounted read-only.
This was because the mount(2) system call ignores mount flags when
creating a bind mount. The solution is to bind mount the directory
first, then remount it as read-only second.
Known bug: submounts still don't get mounted as read-only. That is, if
we're mounting /usr as read-only, and /usr has a submount of /usr/local,
then /usr/local won't get mounted as read-only.