Automatically adjust data transfer policy when backend FTP rejects EPRT/EPSV

[Castaglia]

The scope of this ticket is to make mod_proxy handle the case where the backend FTP server rejects EPSV (or EPRT) commands by automatically adjusting the data transfer policy accordingly. We already do this already by looking at the reported backend features -- however, not all backend servers support FEAT, and even with that, the backend server may still reject such EPSV commands.

I am thinking that mod_proxy can watch for the response codes for failed EPSV and EPRT commands; depending on the type of response code, then the module can automatically adjust the data transfer policy.