#4 support of multiple FTP forward proxies

[Anonymous]

From jlrenard@generali.fr
frox can forward FTP request to another proxy using the
directive FTPProxy.
But if the next proxy is down, no connection is possible.

The ability to define multiples FTPProxy lines in the
config file and frox try each one until one work before
failed the connexion would imporve the availibility of the
FTP proxy chain.