Adapted scripts to simplified script API

This commit takes into account the following renames, plus the fact that
their first two parameters have been dropped:

    npc_message     -> say
    npc_choice      -> ask
    npc_ask_integer -> ask_number
    npc_ask_string  -> ask_string
    npc_trade       -> trade
Adapted to use entity member functions

64 functions were moved and renamed, affecting lots of code.