bus: provide org.freedesktop.systemd1.busname for systemd --user
core: do not read system boot timestamps in systemd --user mode

Before:
  $ systemd-analyze --user
  Startup finished in 2.810s (firmware) + 48ms (loader) + 122ms (userspace) = 122ms

After:
  $ systemd-analyze --user
  Startup finished in 122ms (userspace) = 122ms