kvargs: simpler parsing and allow duplicated keys

kvargs: simpler parsing and allow duplicated keys

Remove the rte_kvargs_add_pair() function whose only role was to check
if a key is duplicated. Having duplicated keys is now allowed by kvargs
API.

Also replace rte_strsplit() by more a standard function strtok_r() that
is easier to understand for people already knowing the libc. It also
avoids useless calls to strnlen(). The delimiters macros become strings
instead of chars due to the strtok_r() API.

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