Implement the EPSV command

[mdirkse]

"The EPSV command requests that a server listen on a data port and wait for a connection."

Spec: https://tools.ietf.org/html/rfc2428#section-3

See also https://www.jscape.com/blog/what-is-the-ftp/s-epsv-command-and-when-do-you-use-it