Instead of always using the temporary file /tmp/libupdatergtk.so generates a new name.

This solves a problem: if the file already existed and the current user
couldn't truncate it the auto update couldn't use the Gtk auto-updater.

In this commit it also deletes the file and avoids creating and closing
the file and opening again.

MD-20923
Uses FileUtils::removeFile() instead of unlink()

MD-20923
Uses tabs for indentation.

MD-20923
Uses std::string

MD-20923
Simplifies creating the char*, avoids using useless std::string

MD-20923
Uses stack-allocated array.

MD-20923
Close the file descriptor in the same method that had been opened.

MD-20923
Deletes non-needed include.

MD-20923
Merge pull request #9 from cpina/MD-20923-fix_fail_ovewrite_file

Instead of always using the temporary file /tmp/libupdatergtk.so generates a unique filename.