Better support for running as auxiliary service manager

[netbsduser]

We should better support running as an auxiliary service manager. Some rough thoughts:

1. General refactoring of the logic in the manager for behaving differently according to run mode - sometimes it uses getpid(), sometimes other things - it's a mess, needs to be cleaned up.
2. Probably wise to add a 'auxiliary' flag to the manager struct rather than a new RunningAs mode.
3. The special targets and services like dbus.socket and dbus.service - need to handle these

[netbsduser]

Relevant areas:
Connecting to the buses:

InitWare/cmd/manager/manager.c

Line 660 in a8fc883

r = bus_init(m, reexecuting || running_as != SYSTEMD_SYSTEM);

Adding D-Bus dependencies:

InitWare/cmd/manager/service.c

Line 1299 in a8fc883

r = unit_add_two_dependencies_by_name(u, UNIT_AFTER, UNIT_WANTS, // FIXME: was dbus_socket

[netbsduser]

Made some good progress on point 1, implemented point 2 as a new flags enumeration for system mode (flags include is-PID1, is-in-container, is-auxiliary) and point 3 by adding a check-dbus-up.service to check if system d-bus is up. Need to consider the possibility of system d-bus failing too?