Split fs_savepath for config files. This is in preparation for
moving the writable paths on *nix to $XDG_DATA_HOME and
$XDG_CONFIG_HOME.
Affected files: config.spec, *.cfg, doomkey, xpkey.
Don't include the lazy precompiled.h everywhere, only what's
required for the compilation unit.
platform.h needs to be included instead to provide all essential
defines and types.
All includes use the relative path to the neo or the game
specific root.
Move all idlib related includes from idlib/Lib.h to precompiled.h.
precompiled.h still exists for the MFC stuff in tools/.
Add some missing header guards.
