Fix the MSDOS build as fallout of latest changes.

 msdos/sedlibmk.inp (allocator.$(OBJEXT), careadlinkat.$(OBJEXT)): Fix
 editing out.
 msdos/sed2v2.inp (IS_DEVICE_SEP): Edit to match ':'.
 (IS_DIRECTORY_SEP, INTERNAL_TERMINAL): Fix Sed command syntax.
 (MSDOS): Define only if undefined, as MSDOS is a built-in macro,
 unless some std= switch to GCC is used.

 src/msdos.c (dos_set_window_size, IT_update_begin)
 (IT_frame_up_to_date, IT_set_frame_parameters): Use FVAR and WVAR
 instead of direct references.