main/hostapd: make initd script more flexible

We don't actually need an explicit start/stop function here as
all the script does is running start-stop-daemon.

Modernise the service by using command/command_args variables,
which also has the side benefit of allowing easy support of
alternative supervisor if e.g. one wants to auto-restart
hostapd on error (which might be useful to ensure it is always
up, avoiding locking oneself out...), it is now as simple
as this snippet in /etc/conf.d/hostapd:
> supervisor=supervise-daemon
> supervise_daemon_args="--respawn-delay 3"