Updates to setuid binaries

[natefoo]

• Generalize name for EL variants (Rocky, etc.)
• In cvmfs_remount_sync, don't use the keys dir to check if a repo exists since keys are in a subdir by default now. Just check if the mount exists and return 0 with a warning if it doesn't, since it probably just means the repo isn't mounted. (EL6 binary is not updated though).
• Add binary for EL8 (although probably the older ones should work fine because of that whole binary compatibility since 1970 thing).