Local file overwrite via predictable symlink path

Link following in FastNetMon Community Edition 1.2.9 and earlier allows local users to overwrite files writable by the daemon. The print_screen_contents_into_file() function truncates the predictable /tmp/fastnetmon.dat path through std::ofstream without rejecting symlinks, while daemonization sets umask to 0 and chmod targets the wrong path variable. Exploitation requires local access and control of the shared temporary path; a root-run daemon lets the attacker redirect writes to root-owned files and make newly created targets world-writable.