Implement user mode "x": hostname cloaking (closes: #102)

Implement user mode "x": hostname cloaking (closes: #102)

When a client has user mode "x" set, its real hostname is cloaked
by substituting it with the server name (as configured in ngircd.conf).

Restricted clients (user mode "r") aren't allowed to change mode "x".

Please note that hostname cloaking is only in effect in server-client
communication! The server still uses the real hostname for its own
logging and for all server-server communication -- therefore all servers
in the network must support user mode "x" to prevent older servers
from leaking the real hostname of a cloaked client!