* gnu/system/dmd.scm (mingetty-service): Add 'stop' field.
(nscd-service): Run with '--foreground'. Change 'stop' to use
'make-kill-destructor'.
(syslog-service): Run with '--no-detach'; change 'stop' similarly.
(guix-service): Likewise for 'stop'.
(static-networking-service): Remove 'sleep' call from 'start'. Change
'stop' to return #f when networking was actually stopped.