Preliminary subuid and subgid support
Fix bug in subuid parsing, add `vagga _userns` command

The `vagga _userns` command is very similar to lxc-usernsexec, but with
vagga flavour. Basically it should replace lxc-usernsexec in builders.
Always build under user namespace (not all backends tested)
Fix debian-simple builder
Fix other image-based builders
Do not double-unshare user namespace
Add first version of _clean command
Using user namespaces for removing old container build artifacts
Add big nice label on process shutdown
Fix debian-debootstrap defaults
Propagate RUST_LOG to builder script
Fix propagation of uid/gid-ranges into provision
Make etc dir for resolv.conf
Better error reporting for non-existent uid/gid mappings
Better debugging of uid/gid ranges