ex-20070109

API changes

* os.unsetenv is now folded into os.setenv

* os.rmdir is now folded into os.remove

* os.dir now elides . and .. entries from the listing

* lock and unlock now appear in the io table

* io.lock and io.unlock return the file object in case of success

POSIX implementation changes

* ENVIRON_DECL should be defined to expand to extern char **environ; on systems which neglect to declare environ when including <unistd.h>

* prepare for dirent metatable

* ex_pipe() refactored

Windows implementation changes

* os.remove must be overridden

* prepare for dirent metatable

* simplify directory iteration by adding POSIX-like abstraction

* ex_pipe() refactored