Manipulating player stats is pretty simple.  The best place to look is
the Living object (you can find it under the Alphabetical List).

All you need to do is call the relevant function in the right object. For
example:

this_player()->set_strength(15);

... would set the current player's strength to 15. If you wanted to add 2
to his current strength:

this_player()->set_strength(this_player()->query_strength() + 2);

Some commonly changed stats, such as hp, have an add_ function:

this_player()->add_hp(-5);

This is more convenient than calling both query and set. Note that you
should be careful about changing stats, since changes are permanent.
If you want temporary penalties, you should probably look into
bonuses/curses instead.
