The config file parser now has a maximum level of inclusions allowed (5)
to detect and prevent recursive inclusions of itself leading to badness.
Bonus: clean up return code handling from the config parser.
Test 774 verifies
Closes#19168
Only actually needed servers should be listed and none is then implied
if no servers are listed.
Outputs a warning if "none" is still set as a server.
Closes#18466
Like when trying to import an environment variable that does not exist.
Also fix a bug for reading env variables when there is a default value
set.
Bug: https://curl.se/mail/archive-2024-02/0008.html
Reported-by: Brett Buddin
Add test 462 to verify.
Closes#12862
