Since we don't have our own signal handler anymore, those two
init functions can be merged into the existing Sys_Init(), which
is already called by common->Init().
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.
Thread return types are different between platforms, and its
probably not a good idea to return something of a different size,
cast the callback and expect it to not crash.
