Set default for key_menu_screenshot to 0.

In #364 I recommended -1 as the default to avoid taking screenshots in
some situations where a key event is posted for key 0. It turns out
that wasn't such a great idea, as -1 is not parsed properly by the
config parser, and is shown as ??-1 in the setup tool. Default to zero
instead, and add guards to prevent screenshots being taken by mistake.

Thanks to Alexandre-Xavier for pointing this out.
Rearrange order of SDL sound startup.

Initialize low-level sound startup (calls to I_InitSound) separately
from the high-level sound startup (S_Init). In particular, make sure
that SDL sound is initialized before the textscreen multiplayer
waiting screen is shown. This is an attempt to fix a bug with sound in
multiplayer games on Windows; calling SDL_QuitSubSystem(SDL_INIT_VIDEO)
(on closedown of the textscreen library) causes subsequent attempts to
initialize audio to fail.

Big thanks go to Alexandre-Xavier (AXDOOMER) for working out a fix for
this bug and James Haley (Quasar) for giving some technical background
about the lack of separation between SDL subsystems. This (hopefully)
fixes #270.