Synopsis:
   $onchannel([<nick> <channel>|<channel>|*]) 
Technical:
   This function returns a list of users currently on the specified channel.
   It will only operate for channels that the client is currently on.
   Additionally, this function can be used to test to see if a given nick
   is on a specifed channel.
Practical:
   This function basically does the same thing as NAMES, except is much
   faster.  However, it only functions on the client's current channels.
   If no channel is specified, the current is used.  Similarly, a "*" will
   expand to the current channel.

Returns:
   list of nicknames on specified channel, or nothing if error

Examples:
   $onchannel()                  shows users on current channel
   $onchannel(*)                 also shows users on current channel
   $onchannel(#foobar)           shows users on #foobar if client is there
   $onchannel(aUser #foobar)     returns true if aUser is on #foobar and
                                 client is there.
Aliases:
   The $chanusers() function is identical in operation to $onchannel().

See Also:
   names(2)

Bugs:
   There is a long-standing bug in many servers that can lead to erroneous
   output from this function.  These servers limit the length of numeric
   353 (RPL_NAMEREPLY) to 510 characters.  When the nickname list exceeds
   this, the server uses multiple 353s.  Unfortunately, they will truncate
   any nickname that would end up exceeding the 510th character.  This
   function uses 353 to generate it's initial nickname list, so it is
   likely that it will return chopped nicknames on large channels.  This is
   an unavoidable server bug.

Other Notes:
   This function will fail on channels with mode +a set (ircd 2.9 servers),
   as those channels do not permit a listing of users.

