Unbreak the MinGW build broken by recent changes in callproc.c

Unbreak the MinGW build broken by recent changes in callproc.c

* src/w32.h (set_process_dir):
* src/w32proc.c (set_process_dir): Change the argument to 'const
char *'.
* src/lisp.h (make_environment_block):
* src/callproc.c (make_environment_block): Now returns 'char **'.
(exec_failed) [DOS_NT]: Remove unused function.
* src/callproc.c (child_setup): NEW_ARGV and ENV are now 'char **'.
Making them 'const' breaks the MinGW build and is not needed for
other platforms.
* src/callproc.c (emacs_spawn): ARGV and ENVP arguments are now
'char *', for the same reason.
* src/process.c (create_process): Adapt to above changes.