[MINOR] introduce structures required to support Linux kernel splicing

[MINOR] introduce structures required to support Linux kernel splicing

When CONFIG_HAP_LINUX_SPLICE is defined, the buffer structure will be
slightly enlarged to support information needed for kernel splicing
on Linux.

A first attempt consisted in putting this information into the stream
interface, but in the long term, it appeared really awkward. This
version puts the information into the buffer. The platform-dependant
part is conditionally added and will only enlarge the buffers when
compiled in.

One new flag has also been added to the buffers: BF_KERN_SPLICING.
It indicates that the application considers it is appropriate to
use splicing to forward remaining data.