updated posix library

[Luiz Henrique de Figueiredo <lhf@...>]

I've updated my posix library. It's available as usual at
        http://www.tecgraf.puc-rio.br/~lhf/ftp/lua/

The main changes are:
 - fixed a bug in putenv
 - fixed a bug in getpasswd
 - added a demo program by John Belmonte

Someone mentioned here in the list that this library does not contain all of
posix. What is missing that could be included? (Note that some of posix is
included in the standard Lua libraries; lposix does not duplicate this.)

Thanks for you feedback.

Enjoy.
--lhf