tiles.rc: let zone population be controlled by tiles.rc

git-svn-id: http://micropolis.googlecode.com/svn/trunk@800 d9718cc8-9f43-0410-858b-315f434eb58c
demand valves: fix bug in demand valve determination

is supposed to be dependent on tax rate, but due to using the wrong
local variable, it was ignoring the tax rate.
This fix will make the economy a little slower.

git-svn-id: http://micropolis.googlecode.com/svn/trunk@801 d9718cc8-9f43-0410-858b-315f434eb58c