Control sound-mute via semfile (ctrl/sound.muted) rather than sbbs.ini

There was a bug with reloading the configuration files in sbbsctrl where the sound button no longer reflected the truth and the sysop's previous click-state of the button was lost. Rather than going through writing the OPT_MUTE flag to the Options fields of all the sections of the sbbs.ini and then re-loading that file as a result, just do like we did with the sysop chat availability: use a semfile. So much simpler.

If anyone ever needs instance-specific muting, we can create/check instance/host-specific mute semfiles then. Doubt that'll happen though.

Also, removed the old sysavail control methods of ntsvcs too.