Validate params of config_get()

[Federico Razzoli]

If we put invalid values for config options, unexpected behavior can happen. So we need to tell the user a clear error message whenever he tries to assign an invalid value.

* dbug, show_err are booleans; valid values: 1, 0, on, off, yes, no (case insensitive)
* out_format: text, html

Valid booleans must be converted to 0 or 1, so we dont need to modify other procedures.

[Federico Razzoli (santec)]

Fixed: rev 32