app/testpmd: add flow item spec handler

app/testpmd: add flow item spec handler

Add parser code to fully set individual fields of pattern item
specification structures, using the following operators:

- fix: sets field and applies full bit-mask for perfect matching.
- spec: sets field without modifying its bit-mask.
- last: sets upper value of the spec => last range.
- mask: sets bit-mask affecting both spec and last from arbitrary value.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Olga Shern <olgas@mellanox.com>