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.