Treat redirection of standard handles on Windows as it is done on Unix.

* job.c (start_job_command) [WINDOWS32]: Compute outfd and errfd
as on Posix platforms, and pass the results to process_easy.
* function.c (windows32_openpipe) [WINDOWS32]: Accept an
additional argument ERRFD and use it for redirecting the standard
error handle passed to the subprocess.
(func_shell_base) [WINDOWS32]: Pass the computed errfd to
windows32_openpipe.
Avoid compilation warnings.

* main.c (find_and_set_default_shell, main) [WINDOWS32]: Declare
variables 'const char *' to avoid compiler warnings.
* job.c (construct_command_argv_internal) [!NDEBUG]: Declare 'end'
and set it only if NDEBUG is not defined, to avoid compiler
warnings.
* makeint.h (ftruncate): Define also for __WATCOMC__.

Copyright-paperwork-exempt: yes.
Fix last commit.