nspawn: add --forward-journal= and --forward-journal-*= options

nspawn: add --forward-journal= and --forward-journal-*= options

Add --forward-journal=FILE|DIR to forward the container's journal
entries to the host via systemd-journal-remote. When specified,
nspawn starts systemd-journal-remote listening on a Unix socket,
bind-mounts it into the container at /run/host/journal/socket, and
passes a journal.forward_to_socket credential pointing to it.

Add --forward-journal-max-use=, --forward-journal-keep-free=,
--forward-journal-max-file-size=, and --forward-journal-max-files=
to configure disk usage limits for the forwarded journal.

Consolidate nspawn's per-machine on-disk state under a single runtime
directory at /run/systemd/nspawn/<machine>/. The container rootdir
mount point moves from /tmp/nspawn-root-XXXXXX to <runtime_dir>/root,
the unix-export directory moves from
/run/systemd/nspawn/unix-export/<machine> to <runtime_dir>/unix-export,
and the journal-remote socket lives at
<runtime_dir>/journal-remote-socket. Update ssh-generator and
ssh-proxy to follow the new unix-export path layout.

Extract fork_journal_remote() into fork-notify.{c,h} as a shared
helper used by both nspawn and vmspawn, replacing vmspawn's
start_systemd_journal_remote().

Extract runtime_directory_make() into path-lookup.{c,h} as a shared
helper used by both nspawn and vmspawn, replacing vmspawn's inline
runtime directory creation logic.

Co-developed-by: Claude Opus 4.6 <noreply@anthropic.com>