Fix the MSDOS build when running under CWSDPMI
* src/msdos.c (the_only_tty_output): Define. * src/msdos.h (the_only_tty_output): Declare. * src/frame.c (make_terminal_frame) [MSDOS]: * src/dispnew.c (init_display) [MSDOS]: Set up f->output_data.tty pointer using the_only_tty_output, before dereferencing the pointer. This prevents crashes with DPMI servers that provide NULL pointer protection.