Enhanced version of the dbus-service-wait executable found at
https://github.com/kozec/unit2openrc/blob/master/dbus-service-wait.c.

Original description:
  Simple executable that waits until service with name passed as argument
  is available and exits with exit code 0.

  If anything fails, exits with code 1.

Changes:
  Fix typo (excepted -> expected)
  Use the session bus if available.
    Remove DBUS_SESSION_BUS_ADDRESS from the environment to force the system
    bus.
  Misc spacing issues
  Use arg.h to follow POSIX utility syntax guidelines
    (more specifically, not require a space)
  Additional arguments are treated as program to exec(3) into.
    Listening for readiness then happens on the background.
  -d option: write newline to an fd (>=3) once a name is acquired.
    With additional arguments, useful for simulating systemd Type=bus services
    under s6 and dinit. For instance, you can write to the `run' file or to
    command= (use 3 as the notify-fd):
      dbus-service-wait -d3 net.connman.iwd iwd