[ccache] Buffer size for IO operations is too small

[Anders Björklund]

> But having to look for various OS/kernel versions of sendfile()? Eww.
> Might as well stick with "splice()", since other main systems like
> have solutions already: Win32 have CopyFile and OS X has copyfile.
> And doing some "advise/allocate" sounded easy, but had pitfalls too.
>
> Here is the end result, in case anyone is interested in a preview:
> https://github.com/jrosdahl/ccache/compare/master...itensionanders:uncompressed
>
> It sounded like a good idea, but needs some actual benchmarks to see
> whether it was actually worth it. Probably should check st_blksize too.

Turns out the OS X version needed another header (<copyfile.h>) to work,
and that it was mostly about HFS metadata. Actually *slower*, for files!

So removed the system versions from the code, only kept the splice().
The fadvise/fallocate only seems to exist on Linux, OS X uses fcntl ?

> The actual I/O can probably be made twice as fast (e.g. for a 1M file)
> Question is whether it makes any real impact of the ccache run time ?

Too much hassle, it seems ? The Linux version might be worth keeping,
but for the rest it seems that the KISS principle applies (as always)

/Anders