Fix chdir, getcwd, run-program on ABCL, add tests.
The JVM doesn't give access to native chdir without some extensions that aren't available with ABCL. There's a user.dir system property whereby some Java programs kind of fake it, but ABCL doesn't play nice with it. Therefore, use *default-pathname-defaults* as our "current directory", and be sure to chdir to it before to run commands with run-program. Try to make the run-program chdir feature work on Windows.