kvargs: add a new library to parse key/value arguments

Copy the code from rte_eth_pcap_arg_parser.[ch], without any functional
modifications, only:
- rename functions and structure
- restyle (indentation)
- add comments (doxygen style)
- add "const" or "static" attributes, remove unneeded "inline"

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>