change SSIZE_MAX to INT_MAX instead of 32767.

change SSIZE_MAX to INT_MAX instead of 32767.

With this change, suggested by Gautam Tirumala, ports for pkgin and
pkg_install are cleaner and so easier to upstream.  Presumably other
ports will be smoother too.

There doesn't seem to be a reason SSIZE_MAX was so small to begin with.