header.pl: Add utf-8 handling into cleanhtml command

header.pl: Add utf-8 handling into cleanhtml command

- existing cleanhtml command does not handle diacritical charcters such as umlauts, acute,
   grave and circumflex accents.
- In bug 12395 the problem was resolved by adding decode before and encode after the
   cleanhtml command in dns.cgi
- Suggestion from @Michael Tremer was to add the decode and encode sections into the
   actual cleanhtml subroutine in header.pl
- This patch submission is the execution of that suggestion.
- This will ensure that whenever cleanhtml is used for any remark in a WUI page it will
   handle diacritical charcters.
- Tested out on my vm testbed system and confirmed to be working when cleanhtml has the
   encode and decode lines.
- Combined with this patch is another one that changes the dns.cgi to remove the decode
   and encode entries added into the cgi code.

Suggested-by: Michael Tremer <michael.tremer@ipfire.org>
Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>