feat: make MODE Z configurable

[drakkan]

This is a non-standard FTP extension and is not supported by major clients:

https://trac.filezilla-project.org/ticket/2977
https://winscp.net/forum/viewtopic.php?t=25351

Additionally, it could potentially increase the attack surface, although I have not been able to fully verify this:

$ lftp ftp://127.0.0.1:2121
lftp 127.0.0.1:~> debug on;
lftp 127.0.0.1:~> set ftp:use-mode-z yes;
lftp 127.0.0.1:~> login a password
lftp a@127.0.0.1:~> ls
**** zlib inflate error: incorrect header check
ls: Errore fatale: zlib inflate error: incorrect header check
lftp a@127.0.0.1:/> quit

Do you know of any compatible client? Which client has it been tested with?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.15%. Comparing base (ca491b6) to head (2a52679).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #604      +/-   ##
==========================================
+ Coverage   87.14%   87.15%   +0.01%     
==========================================
  Files          12       12              
  Lines        1828     1830       +2     
==========================================
+ Hits         1593     1595       +2     
  Misses        157      157              
  Partials       78       78              

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