Use "ssize_t" not "long" in max_stack_depth-related code.

Use "ssize_t" not "long" in max_stack_depth-related code.

This change adapts these functions to the machine's address width
without depending on "long" to be the right size.  (It isn't on
Win64, for example.)  While it seems unlikely anyone would care
to run with a stack depth limit exceeding 2GB, this is part of a
general push to avoid using type "long" to represent memory sizes.

It's convenient to use ssize_t rather than the perhaps-more-obvious
choice of size_t/Size, because the code involved depends on working
with a signed data type.  Our MAX_KILOBYTES limit already ensures
that ssize_t will be sufficient to represent the maximum value of
max_stack_depth.

Extracted from a larger patch by Vladlen, plus additional hackery
by me.

Author: Vladlen Popolitov <v.popolitov@postgrespro.ru>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/1a01f0-66ec2d80-3b-68487680@27595217