Use a single vnet_pcap_t in vlib_global_main, specifically to support
unified tracing
Update sphinx docs, doxygen tags
Type: refactor
Ticket: VPP-1776
Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: Id15d41a596712968c0714cef1bd2cd5bc9cbdd55
(cherry picked from commit 33909777c63712ca397165cd92e7cc62208eb5c8)