Improve error reporting in the Windows port when env size is too large.

Improve error reporting in the Windows port when env size is too large.

w32/subproc/misc.c (arr2envblk): Compute and return the size of
the environment passed to child process.

w32/subproc/sub_proc.c (process_begin): If the call to
CreateProcess failed with EINVAL, and the required environment
size was larger than 32KB, assume it's a Windows XP limitation,
and display an error message to that effect.

w32/subproc/proc.h (arr2envblk): Update prototype.

Copyright-paperwork-exempt: yes