pg: set vnet buffer flags in pg streams

pg: set vnet buffer flags in pg streams

See .../extras/pg/checksum_offload.pg for a nontrivial example, which
deliberately sets bogus ip and udp checksums in the generated packets,
then fixes the mess with (software emulated) hardware checksum
offload.

Validated via "pcap dispatch trace on max 1000 buffer-trace pg-input 100".
Packets stuffed into loop1-output have the configured bogus ip and udp
checksums. vnet_calc_checksums_inline(...) fixes the checksums, which
are correct when packets visit loop1-tx.

The packet generator is a dumb robot in this regard. If you ask for a
ridiculous flag combination - example: ip4 and ip6 - your mileage may
vary.

Type: feature

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: I6d9e790f373bcd9e445a89113ca8e4c8f9aa9419