af-packet: finalize code

af-packet: finalize code

This patch handles the end of AF_PACKET socket support work. It
provides conditional compilation, autofp and single runmode.

It also adds a 'defrag' option which is used to activate defrag
support in kernel to avoid rx_hash computation in flow mode to fail
due to fragmentation.

This patch contains some fixes by Anoop Saldanha, and incorporate
change following review by Anoop Saldanha and Victor Julien.

AF_PACKET support is only build if the --enable-af-packet flag is
given to the configure command line. Detection of code availability
is also done: a check of the existence of AF_PACKET in standard
header is done. It seems this variable is Linux specific and it
should be enough to avoid compilation of AF_PACKET support on other
OSes.
Compilation does not depend on up-to-date headers on the system. If
none are present, wemake our own declaration of FANOUT variables. This
will permit compilation of the feature for system where only the kernel
has been updated to a version superior to 3.1.