Fix the MS-Windows build due to introduction of B_.

Fix the MS-Windows build due to introduction of B_.

 dired.c (directory_files_internal):
 fileio.c (Finsert_file_contents):
 insdel.c (prepare_to_modify_buffer):
 xdisp.c (pos_visible_p):
 s/ms-w32.h (MODE_LINE_BINARY_TEXT):
 w32fns.c (Fw32_shell_execute, Fx_show_tip, x_create_tip_frame):
 Use B_ for the MS-Windows build.

 From Tom Tromey:
 buffer.c (init_buffer_once, syms_of_buffer): Use B_ in DOS_NT case.