Change the default stack size to safe values on OpenBSD.

With this patch racket will never grow beyond of the defaults limits of
the OS and also it doesn't limit the stack size to a fixed value.

Merge to v5.3.4
Unix: initialize `current-directory' from the PWD environment variables

... when PWD is defined and when it refers to the same directory
as the result of getcwd().

A shell sets PWD before starting Racket to communicate a preferred
way of referring to the current directory, which may involve soft
links that are not reflected in getpwd().
racket/system: make `system', etc., set PWD by default

That is, make `system' behave like a shell.