cat: GNU has better throughput without splice() code path

[oech3]

$ truncate -s 9PB /tmp/huge
$ strace -o /dev/null -e inject=splice:error=ENOSYS cat /tmp/huge | pv>/dev/null
^C08GiB 0:00:02 [ 521MiB/s]
$ strace -o /dev/null -e inject=splice:error=ENOSYS gnu-9a3479f1e/cat /tmp/huge | pv>/dev/null
^C72GiB 0:00:01 [1.72GiB/s] 

This is important on non-Linux platforms.

[oech3]

raising buf size to N MiB improved to ~ 1.0GiB/s. But still far from 1.72GiB/s.