additional validation of the --cheaper value at startup

Fixes #152

The way current ```--cheaper``` (and probably some other dynamic options) work causes UWSGI_OPT_CHEAPER to be defined if user sets it to any value, so even ```---cheaper 0``` will enable it. ```if (op->flags & UWSGI_OPT_CHEAPER)``` check will pass in ```core/utils.c``` and ```uwsgi.cheaper``` will be set to 1.
The simplest fix for cheaper is to add another validation (and so I did), but there are probably other options that suffers from this, so maybe we need a better and more global change.