fix config::get_diff
use sides_.size() instead of MAX_PLAYES

we usualy want to use that number to index the sides_ vector so
comparing against sides_.size() makes more sense.
We now also use side_index for the index of the side in sides_ and
side_num for the value of side_cfg["side"]
I also put some additional checks.
limit variable scopes

knowing that those varaiables aren't used later down makdes the code
easier to understand.
don't limit gold and income in mpconnect if not neeeded

partly fixes http://gna.org/bugs/?22485.
don't print blank attribute values

blank values have no string representation, the are more temporary
values created becasue someone wanted to put something in them but then
thought different.

writing and reading them caused then to be converted to empty string
values which is something diffrerent.