cksum family: read ahead file

[oech3]

Most people does not calc checksum many times for the same file.
So page caches are missing when cksum is called.

[oech3]

cfg is complex

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/retry (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/pr/bounded-memory is now being skipped but was previously passing.

[oech3]

hyperfine disallows dropping caches

[cakebaker]

Thanks!

[oech3]

I might not do that for everything, but it seems GNU sometimes call it

$ cat /etc/pacman.conf|strace -qqq -e trace='/fadvise' gnu9.11/wc -c
fadvise64(0, 0, 0, POSIX_FADV_SEQUENTIAL) = -1 ESPIPE (Illegal seek)
535