limit hostname-char-set input size

Fuzzer reveals that the C++ regex library blows on silly input for hostname-char-set. As it uses regular expressions no large values are expected and it is not a problem to limit the input to something reasonable.

Note that regular expressions can lead to hyper-exponential computing time so there is a potential attack vector using them. Currently the expression itself is in the configuration so it is just another way for an administrator to shoot on the foot...