schroot backend: pass --session-id to adt-virt-schroot.

This makes it easier to debug test suite that leave rogue schroot
sessions behind.
rename `debci` binary to `debci-batch`.

`debci` stays reserved for providing a entry point binary like `git`
that will invoke subcommands that are kept out of $PATH to avoid
cluttering the namespace.
Resolve actual config dir to list default in usage
debci-batch: make batch locking suite- and architecture-specific
debci-batch: add --offline and --online options

--offline will prevent new test runs from being started, and --online will
revert to normal operation.