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/
(cherry picked from commit 1d4fb5882fcaa898fe740ea983e9676a51d1b992)
ftp: Relax requirements for EPSV and EPRT

Fix unit tests after 1d4fb58.

(cherry picked from commit af625e3cf88fe40a1d009dc46c1c6018047511bd)
Merge pull request #1141 from gbehrmann/fix/2.11/rb7537

ftp: Relax requirements for EPSV and EPRT