Synopsis:
   $servernick(<refnum>)

Technical:
   This function returns the client's current nickname on a given server.

Practical:
   This function is good when using multiple servers with possibly different
   nicks.

Returns:
   nickname on server <refnum>

Examples:
   $servernick(0)               always the same as $N
   $servernick(2)               client's nick on server 2