[BUGFIX] Remove dependency to iconv lib in form

The form extension uses iconv_strlen to determine
the string length, which fails when this method
is not present in PHP.

Use the CharsetConverter class instead.

Fixes: #42990
Releases: 6.0

Change-Id: Ibf7e82c85bd808f036c7d0715f55d6f1f9e5b8d5
Reviewed-on: http://review.typo3.org/16449
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel