Add a wrapper for splice()

[Arcterus]

On Linux, we should try to use splice() rather than the standard read/write stuff for operations involving pipes. This mostly affects sh.

[Arcterus]

Depending on how far back we need to support we could use copy_file_range() for things as well (it doesn’t require one of the file descriptors to be a pipe, but it requires Linux 4.5).

[Arcterus]

It sounds like fs::copy() may already do this, so we might want to investigate a little more before deciding to add this.