app/testpmd: add rxoffs commands and parameters

app/testpmd: add rxoffs commands and parameters

Add command line parameter:

--rxoffs=X[,Y]

Sets the offsets of packet segments from the beginning of the
receiving buffer if split feature is engaged. Affects only the
queues configured with split offloads (currently BUFFER_SPLIT
is supported only).

Add interactive mode command, providing the same:

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

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

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