bug#13070: Use putenv+unsetenv instead of modifying environ directly

[Paul Eggert <eggert@cs.ucla.edu>]

On 12/08/2012 03:42 AM, Eli Zaretskii wrote:
> Shouldn't we refrain from signaling memory_full when errno is EINVAL?
> I'd suggest an eassert in that case.  memory_full will emit a
> misleading diagnostic.

errno cannot be EINVAL, at least not on a POSIXish host:
all strings are allowed as arguments to putenv.
If Microsoft platforms are different it would
make sense to put in an eassert, in w32.c presumably.

I did see a minor problem in the w32.c implementation of unsetenv:

  /* MS docs says an environment variable cannot be longer than 32K.  */
  if (name_len > 32767)
    {
      errno = ENOMEM;
      return -1;
    }

unsetenv should return 0 in that case, not -1, since
the variable cannot possibly be in the environment.
This bug doesn't affect Emacs so it's not a pressing one.

I committed the patch as trunk bzr 111158 and am marking
this bug as fixed.