supervisor: Use pivot_root and MS_DETACH to detach from the outside world
Rearrange filesystem layout to use no chroots and a single pivot_root

The supervisor process retains a capability: its current directory
is an isolated bind-mount of the grain directory, despite the fact
that the grain directory is not mounted anywhere.  Only the "sandbox"
subdirectory is mounted.
Use user namespaces

On new enough kernels, sandbox-supervisor now works without being
setuid root.  I know of two regressions:

 * --proc is broken when user namespaces are used.  Fixing this is possible
   but tricky.

 * Running sandbox-supervisor as root with user namespaces fails.  This
   happens because, once root has entered the namespace, it has insufficient
   privilege to map anyone else into the namespace.  This should be
   straightforward to fix.
sandbox-supervisor: Remove non-userns support and all uid and gid code

This now requires CONFIG_USER_NAMESPACE.  It is no longer safe to install
setuid root.  It is probably unwise to run it as root.  The code
simplification should be worth it.
sandstorm-supervisor: Fix --proc
Update to latest Meteor and other dependencies.
Apparently Meteor is now nonsensically rewriting URLs in CSS.
FUSE -> Cap'n Proto bindings. Read-only for now.
Merge branch 'master' of github.com:kentonv/sandstorm
Improve FUSE caching and other tweaks. PS this fuse stuff is for a build tool.
WIP tool to run apps in 'dev mode' and auto-detect dependencies.
Make it easier to install custom builds, e.g. for local testing.
Continue work on 'spk dev'.

App is now being FUSE-mounted into the daemon's namespace and the front-end is being notified; all that's left is for the front-end to actually react. But I need a better way to do front-end development while connected to a real Sandstorm installation...
Make it easier to run a dev front-end against an installed Sandstorm instance.
'spk dev' successfully registers dev apps with server. Now just need to capture output.
Easy port mostly working.
Don't care if deleting old versions on startup fails.
Detect mongod binary and print warning about database size.
Add checks to spk tool to block overly-large apps.
Enable authentication on MongoDB, with password stored in a file readable by the server's group.

Previously we just left the DB fully-accessible to anyone on localhost, which is probably fine for most users, but since we claim that the Sandstorm instance is only accessible to people in the corresponding group, we really ought to protect it as such.