Partial cleanup of variable shadowing warnings

I went through and cleaned up a bunch of warnings raised by -Wshadow (which is cases where a variable is declared with the same name as another variable - potentially pretty bad, if unintended).

Unfortunately, I found a massive 8000-line function in mapgen.cpp that seemed to be inconsistent in whether it used function-level or block-level temporary variables for x and y. It seems like some cases may be assuming that x,y were already initialized, but... I really can't make heads or tails of long-distance interactions in that function. Needs a separate cleanup before I can continue, I think.

Another consideration: two places involved a parameter `z` (the coordinate) shadowing `game::z` (the monster list). I changed the coordinate to `z_coord`, due to that involving fewer changes, but this might be a good impetus to rename `game::z` to something sane (`game::monlist`, maybe?).