daemon: make --inetd and --detach incompatible

Since --inetd makes main return with the result of execute() before
daemonize is gets called, these two options are already incompatible.

Document it, and add an error if attempted.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>