pg: cli improvement

pg: cli improvement

1. show packet-generator verbose, display all edit-groups and hdr-size
2. unformat_pg_payload, always mark payload hdr-size as 0
3. packet-generator now can change rate/limit/size at runtime
4. validate_stream checks buffer min-size/max-size against all edit
   groups' header size
5. remove incorrect max packet size limit check in validate_stream(...)

Change-Id: Ic45e4f2b98bc0fd7330e0b480dd677fa3c69a677
Signed-off-by: Kingwel Xie <kingwel.xie@ericsson.com>
Signed-off-by: Dave Barach <dave@barachs.net>