Build mkgmap/mkgmap #1266

- fix long standing errors in the code that reads extended data (introduced with r2546) - method fetchPointsCommonExtType expected an offset for end but caller passed the size. Proably never did anything useful. Renamed to fetchPointsExtType (there is only one type) and fixed the error. - remove wrong copy+paste code which tried to read labels from NET for extended type lines. Not sure what it did with files that were not produced by mkgmap. Expect more data in overview map if the ovm_*.img files contain points with extended types (0x1xxxx).