rc-mount: make timeout invocation compatible with busybox timeout
rc-mount: make timeout invocation compatible with busybox timeout

Busybox timeout does not support the `-k` flag. As such, invoking fuser
from do_unmount never worked without this patch. This went unnoticed as
standard error is redirected to /dev/null. This patch fixes this by
simply removing the incompatible `-k` flag.

[Ariadne: the `-k` is redundant anyway, since we are sending the KILL
 signal to begin with.]

OpenRC/openrc,OpenRC/openrc,OpenRC/openrc,williamh/openrc,williamh/openrc,williamh/openrc