Return 502 on the REIN command

[sudhirj]

We have a system that we're working with that tries to send a REIN and bombs when it gets a 500 back. I believe it expects a 502, which makes more sense semantically - it's unsupported, not a syntax error. VSFTPD also sends back a 502, which is why this system seems to work everywhere else.

Will see if I can raise a PR to send a 502 on REIN. Will add it into the handle_misc file, don't think this falls into any of the other defined categories.