systemd,nspawn: use extended attributes to store metadata

All attributes are stored as text, since root_directory is already
text, and it seems easier to have all of them in text format.

Attributes are written in the trusted. namespace, because the kernel
currently does not allow user. attributes on cgroups. This is a PITA,
and CAP_SYS_ADMIN is required to *read* the attributes. Alas.

A second pipe is opened for the child to signal the parent that the
cgroup hierarchy has been set up.