Set or clear a file segment lock according to the lock description pointed to by the third argument, arg, taken as a pointer to a Mono.Unix.Native.Flock. FcntlCommand.F_SETLK is used to establish shared (or read) locks (LockType.F_RDLCK) or exclusive (or write) locks, (LockType.F_WRLCK), as well as remove either type of lock (LockType.F_UNLCK). If a shared or exclusive lock cannot be set, Syscall.fcntl returns immediately with Error.EAGAIN.