[no sq.] Iconv fixes and improvements

[sfan5]

first: fixes #14463 and also cleaner in general
second: after seeing iconv_open appear in my flamegraph I checked the docs and realized we can reuse it.

To do

This PR is Ready for Review.

How to test

1. check that it compiles
2. check that string conversion works (we have an unittest)

[JurajVajdaIRobot]

Opens fine for me (OSx)

Before:
5.6.0, 5.7.0, 5.8.0 open up the same (broken) way, 5.6.1 opens up without issue (all downloaded app files)

After:

NOTE:
opening minetest worked fine for all of them when opening through terminal, e.g. open /Applications/Minetest-5.8.0.app