Remove audio(9) speaker_ctl(), let open() handle speakers where needed

Remove audio(9) speaker_ctl(), let open() handle speakers where needed

Only five legacy half-duplex hardware drivers require this function to
change between playing and recording:
	i386: ess(4), gus(4), pas(4), sb(4)
	luna88k: nec86(4)

If defined, it is always called early in audio_open(), so just move the
call from audio(4) to each hardware driver's open() handler.

SPKR_ON/OFF remain defined to leave driver-specific code unchanged.

Further cleanup (unchecked speaker_ctl() return values,
FWRITE -> AUMODE_PLAY -> SPKR_ON dances, etc.) can happen later.

Builds fine on i386.
OK ratchov