libtracker-common: There is no longer an os-depedent module/API

This means:

- all tracker_spawn*() API is removed, now using g_spawn*()
- the mplayer extractor has been removed (no one uses it anyway)
- the setrlimits() API has also been removed (only tracker-extract uses it)
- many includes have been cleaned up (this is a really old header)
- strnlen() definitions (for OS' without this API) are defined locally (only 2
  cases).