configs/releng/packages.x86_64: add fatresize, gpart and tmux

• fatresize is a utility to resize FAT filesystems using libparted.
• gpart is partition table rescue/guessing tool. It's the only one from https://wiki.archlinux.org/index.php/Partitioning#Recovery that's not included in releng's packages.x86_64.
• tmux is a terminal multiplexer. An alternative for those that don't like GNU Screen. Requested in FS#68252.