Adjust log messages for invalid and spoofed prefixes

Now invalid prefixes aren't logged no more when originating from an other
server (besides in debug mode), and spoofed prefixes are correctly logged
using LOG_WARNING (from an other server) or LOG_ERR (from a client) levels.

In addition, the log message texts have been adjusted to better reflect
what will happen: commands with invalid prefixes are ignored and logged,
commands with spoofed prefixes will result in the client being disconncted
(regular users) or the command being ignored (other servers).

This cleans up logging of commands related to already KILL'ed clients.
New function IRC_KillClient() to kill clients

The old local function Kill_Nick() in irc.c has been an ugly hack. This
patch implements a generic function for killing clients.

Adjust all callers of Kill_Nick() and respect the return code!
Don't forward KILL commands for unknown clients
Actually KILL clients on GLINE/KLINE

Kill all clients that match a new GLINE/KLINE mask and genrate apropriate
KILL commands. These KILL commands can be superfluous, but are required
when the IRC Operator isn't allowd to set remote G-Lines or if there are
older servers in the network that don't kill clients on GLINE/KLINE.

Closes bug #156.