process_input - provide a hook for the MudOS driver to send a copy of
what the player is typing to the player object.
string process_input( string );
If process_input is present in the player object, then the MudOS driver
parser will send it a copy of each line the player types.
Process_input makes it easy to implement features like command history
and protection against wizcuffs. process_input can also modify the
player input before it is parsed. If process_input returns a non-zero
string, that string is used by the driver instead of what the player