flow: separate parsing and pretty printing

This will allow creating a library of parsers and printers
in the event that other consumers want to use the parsers.

Now flow.c is just the CLI interface to the parsers. Another
patch needed to unify and rename exposed functions.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
flow: add handlers for structs to netlink

This adds handers for structs to netlinnk. These are useful
in flow when doing sets but also when using flow as an IPC
which is useful for debugging. And working with user mode
drivers while they are being ported into the kernel.

Plus a fix for PID. Note I'll need to smash all these
commit into something coherent before submitting anyways

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>