vat: unload unused vat plugins

vat: unload unused vat plugins

If the corresponding vpp plugin is absent, return a non-zero
clib_error_t * from vat_plugin_register ("xxx plugin not loaded"). The
vat plugin calls dlclose on the vat plugin, and it disappears.

Depending on the plugin configuration, this can reduce the vpp virtual
size by several gigabytes.

Added a VAT_PLUGIN(<plugin-name>) macro to vat_helper_macros, clean up
boilerplate vat_plugin_register() implementations. Fixed a number of
non-standard vat_plugin_register methods.

Type: refactor

Change-Id: Iac908e5af7d5497c78d6aa9c3c51cdae08374045
Signed-off-by: Dave Barach <dave@barachs.net>