Convert source file with main() to freeserf.cc

It is generally needed for the source file containing main() to be a C++ file
even if other files are plain C. This change thus makes it possible for
other files to be migrated to C++ as needed.
freeserf.cc: Indent to C++ coding style
freeserf.cc: Convert casts to C++ style casts
Merge pull request #160 from jonls/main-c++

Convert freeserf.c to C++ style file
Abstracting graphics and data source

Set apart of basic and common functionality for graphics and data
source.