Simplify D1X hostage handling
Make briefing->message const
Move common/arch/sdl/digi_mixer_music.c -> common/arch/sdl/digi_mixer_music.cpp
Move common/arch/win32/messagebox.c -> common/arch/win32/messagebox.cpp
Simplify D1X SPREADFIRE truncation
Remove obsolete descent_critical_error variables

These appear to be for reporting serious errors from DOS.  Commit
1de5349 removed the ability to set them to anything other than 0, so
remove them.