ftp: Relax requirements for EPSV and EPRT

Since using EPSV without delayed passive forces the use of a data channel
proxy, we have rejected use of RFC 2428 commands over IPv4. This requirement
can however be relaxed: EPRT can be used over IPv4 without proxying, and
EPSV can be used over IPv4 without proxying when delayed passive is enabled.

The patch has the potential to extend compatibility with clients and I hence
request it to be backported to 2.10.

Target: trunk
Request: 2.11
Request: 2.10
Require-notes: yes
Require-book: yes
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: https://rb.dcache.org/r/7537/