OPTIM/MINOR: make it possible to change pipe size (tune.pipesize)

OPTIM/MINOR: make it possible to change pipe size (tune.pipesize)

By default, pipes are the default size for the system. But sometimes when
using TCP splicing, it can improve performance to increase pipe sizes,
especially if it is suspected that pipes are not filled and that many
calls to splice() are performed. This has an impact on the kernel's
memory footprint, so this must not be changed if impacts are not understood.