sound: Allocate 2 channels for UI sounds and possibly more in the future

This reduces the sound effects (SOUND_FX) channel allocation from 21 to
20 and allocates 2 instead of 1 channel for non-bell, non-timer UI use.

Requested by iceiceice to deal with a MP UI sound issue.

(Backported e39797b6ad615d4f619751a6ec6510ca716f5811 from master.)

Conflicts:
	changelog