Refactor map::shoot() to use string-based terrain id instead of oldter().

[kevingranade]

It was making stairs evaporate when you shot them. Reported by Ember on the forums.
Who knows what else it was doing.
It LOOKS extensive, but all I really did was take:

switch( oldter())
    old_t_foo:
        // do stuff;
    break;

and turn it into:

if( 0 == terrain.id.compare("t_foo") ) {
    // do_stuff;
}

Fixes #6192