[mapplauncherd] Add full path to argv[0] of invoked process

This allows the application to look up its installation prefix (e.g.
/usr) by just looking at argv[0] and calculating relative paths from
there (previously one would need to pass the full path to invoker).
Merge pull request #12 from thp/argv0-full-path

[mapplauncherd] Add full path to argv[0] of invoked process