[Next] Some cleanup of the code

[ygerlach]

This is a continuation of #333

Each commit should work independently, but i haven't tested that. So if some commit is not fitting, we can just drop it.

What i did:

• removed dependency to which and made check if a program is available simpler (use more cmd_exists and replace get_cmd_path)
• removed dead code
• fixed some compiler warnings
• simplified code
• removed dependency to chmod and just use the syscall it self
• removed dependency to chown and just use the syscall it self
• only update the control file if the description was changed (Should fix Hang while editing comment #426)