app/testpmd: add rxpkts commands and parameters

app/testpmd: add rxpkts commands and parameters

Add command line parameter:

--rxpkts=X[,Y]

Sets the length of segments to scatter packets on receiving if split
feature is engaged. Affects only the queues configured with split
offloads (currently BUFFER_SPLIT is supported only).

Add interactive mode command:

testpmd> set rxpkts (x[,y]*)

Where x[,y]* represents a CSV list of values, without white space.

Sets the length of segments to scatter packets on receiving if split
feature is engaged. Affects only the queues configured with split
offloads (currently BUFFER_SPLIT is supported only). Optionally the
multiple memory pools can be specified with --mbuf-size command line
parameter and the mbufs to receive will be allocated sequentially
from these extra memory pools.

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>