environment, for the case that child_setup signals an error.
When mirroring slashes in DOS shell commands, don't fail when
argv[2] has embedded blanks.
+ (Fmsdos_set_mouse_buttons, mouse_setup_buttons): New functions.
+ (syms_of_msdos): Defsubr Fmsdos_set_mouse_buttons.
+ (dos_ttraw): Call mouse_setup_buttons.
* callproc.c (child_setup) [MSDOS]: malloc pwd_var instead of
using alloca; free it after run_msdos_command returns.