Send "fake '*' key" in "MODE -k" replies

Send "fake '*' key" in "MODE -k" replies

According to RFC 2812 3.2.3 "Channel mode message" and the examples
there, it looks like clients should use "MODE -k <key>" to unset channel
keys; and that's how other servers and services behave and do expect it.

(But please note that this is NOT the case for "MODE -l"!)

In the end, it doesn't make sense to specify a key when UNsetting it at
all, and different services behave diffrently when clients do not send
the currently set key to unset it - some ignore such calls, for example!

But this implementation is quite relaxed, it accepts any key when
unsetting channel mode "k" and even accepts no key at all. But the reply
will always include an "*" character for every "-k" parameter.