Code review needed for 6519647

[Jean-Christophe Collet]

These changes provide 2 things:
- a minor bug fix for 6519647  (FTP PUT operation through HTTP proxy)
- an overhaul of the FTP code and laying the bases of a future public 
FTP Client API.
That's why the code is separated between sun.net.ftp and sun.net.ftp.iml
Ultimately, what is in sun.net.ftp will become public either in java.net 
or java.net.ftp

http://cr.openjdk.java.net/~jccollet/6519647/webrev.0/