pipeline: add header look-ahead instruction
pipeline: add header look-ahead instruction

Added look-ahead instruction to read a header from the input packet
without advancing the extraction pointer. This is typically used in
correlation with the special extract instruction to extract variable
size headers from the input packet: the first few header fields are
read without advancing the extraction pointer, just enough to detect
the actual length of the header (e.g. IPv4 IHL field); then the full
header is extracted.

Signed-off-by: Cristian Dumitrescu <25468b8a3698eeb3c368247f8635924bb4a1ce2c@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk