"Silence" sound effects by setting volume to 1% instead of 0%

Fixes a major interaction issue between [fade_out_sound_effects] and
[remove_sound_source] resulting in sound source channels not being
halted (because SDL_mixer is bananas) and thus looping sound sources
"resuming" as soon as volume is set back to 100% with either
[fade_in_sound_effects] or [reset_sound_effects].

This is very inconvenient, but a fix in Wesnoth's sound code doesn't
seem particularly feasible at the moment due to limitations in what
SDL_mixer 1.2.x exposes through its API. The most trivial fix would be
to have sound::reposition_sound() use Mix_HaltChannel() instead of
Mix_FadeOutChannel() when silencing a sound source (distance ==
DISTANCE_SILENT), but that'd kind of defeat the purpose of the fade-out.

... Why am I writing down notes for mainline engine hacking here?