Feature request: Make head and tail use ioctl_ficlonerange

[Vít Novotný]

Hello,

with the increasing support for copy-on-write (i.e. reflink) in modern
filesystems, it seems reasonable for head(1) and tail(1) to try calling
ioctl_ficlonerange(2) where available before copying.

This way, a user can perform the following sequence of commands in bash
to obtain a single combined file without any copying:

    head -n 123 first-file   > combined-file
    head -n 456 second-file >> combined-file
    tail -n 789 third-file  >> combined-file

At first, I considered creating a separate tool for this use case, which
would be called in the following manner:

    reftk A=first-file B=second-file C=third-file cat A1-123 B1-456 C=789 
output combined-file

Then it dawned on me that this is perhaps something coreutils should be
able to do. If you think this feature request is sound, I can provide
the patches.

Best Regards,
Vit Novotny

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