activate: add a new switch --inetd to enable inetd-style socket activation

activate: add a new switch --inetd to enable inetd-style socket activation

Previously, using --accept would enable inetd-style socket activation in addition to per-connection operation. This is
now split into two switches: --accept only switches between per-connection or single-instance operation. --inetd
switches between inetd-style or new-style fd passing.

This breaks the interface of the tool, but given that it is a debugging tool shipped in /usr/lib/systemd/ it's not
really a public interface.

This change allows testing new-style per-connection daemons.