-Fix (#172): annoying "unable to create more" when trying to build units

... but only for the human player.

the value for the maximum unit count is written from the scenario
config.
however it's not consistent at all since you can always get more
units than the maximum by using the starport, or by destroying
a building (will create soldiers).
so in the end, this check is nothing but annoying for the player.

closes #172