shared: move machine registration to shared machine-register.{c,h}

Move register_machine() and unregister_machine() from
vmspawn-register.{c,h} into shared machine-register.{c,h} so both
nspawn and vmspawn can use the same implementation.

The unified register_machine() uses varlink first (for richer
features like SSH support and unit allocation) with a D-Bus
RegisterMachineWithNetwork fallback for older machined. The
interface adds a class parameter ("vm" or "container") and
local_ifindex for nspawn's network interface support.

The unified unregister_machine() similarly tries varlink first
(io.systemd.Machine.Unregister) before falling back to D-Bus.

Both register_machine() and unregister_machine() only log at debug
level internally, leaving error/notice logging to callers.

Add register_machine_with_fallback() which tries system and/or user
scope registration based on a RuntimeScope parameter
(_RUNTIME_SCOPE_INVALID for both), and
unregister_machine_with_fallback() as its counterpart. Both use
RET_GATHER() to collect errors from each scope.

Make --register= a tristate (yes/no/auto) defaulting to auto. When
set to auto, registration failures are logged at notice level and
ignored. When set to yes, failures are fatal.

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

shell-completion/bash/systemd-nspawn

@@ -131,7 +131,7 @@ _systemd_nspawn() {
                 comps=''
                 ;;
             --register)
-                comps='yes no'
+                comps='yes no auto'
                 ;;
             --network-interface)
                 comps=$(__get_interfaces)