dhcp: vat support for the dhcp_client_dump API

dhcp: vat support for the dhcp_client_dump API

Also: permanently solve ordering issues with the vpp builtin vat
plugin loader, by explicitly loading vat plugins once we're sure that
all data plane plugins have registered their APIs / API message
handlers.

Fix compilation / link errors when the vpp builtin vat plugin loader
is disbled by cmake configuration.

Type: fix

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: Id553c63ae860ebfc196c5ad4b55c19e08fec2c9e
Signed-off-by: Ole Troan <ot@cisco.com>