/*
# sendfile TODO

    187 in x86.

        man sendfile

    Transfers data from one FD to another, without passing through userspace as in a read write pair.

    This is therefore the most efficient way to copy one file into another.

    Comparison to other methods and implementation:
    http://stackoverflow.com/questions/10195343/copy-a-file-in-an-sane-safe-and-efficient-way
*/