general-functions.pl: validdomainname misinterprets RFC1035

The function validdomainname checks that each part of a domain name is at least
2 characters in length, but RFC1035 only makes a restriction on a "label" being
at most 63 characters in length. This change allows reverse DNS zones like
2.168.192.in-addr.arpa to be added to the DNS forward configuration, which was
incorrectly prevented before.

Signed-off-by: Axel Gembe <ago@multipixs.com>