nspawn: rename --user= to --uid= and repurpose --user/--system for runtime scope

nspawn: rename --user= to --uid= and repurpose --user/--system for runtime scope

Rename nspawn's --user=NAME option to --uid=NAME for selecting the
container user. The -u short option is preserved. --user=NAME and
--user NAME are still accepted but emit a deprecation warning. A
pre-parsing step stitches the space-separated --user NAME form into
--user=NAME before getopt sees it, preserving backwards compatibility
despite --user now being an optional_argument.

Repurpose --user (without argument) and --system as standalone
switches for selecting the runtime scope (user vs system service
manager).

Replace all uses of the arg_privileged boolean with
arg_runtime_scope comparisons throughout nspawn. The default scope
is auto-detected from the effective UID.

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