Previously, on a typical setup without "console=ttyS0" or similar in
'kernel-arguments', the 'term-console' Shepherd service would always be
marked as failing to start. This is undesirable because it raises a
false alarm: the service is expected to do nothing in this case.
This patch instead marks it as succeeding and logs a message explaining
it's doing nothing.
* gnu/services/base.scm (agetty-shepherd-service): In 'start' method,
succeed when TTY is #f and print a message.
