Plugin infrastructure improvements

Plugin infrastructure improvements

This patch replaces requirement for vlib_plugin_register function
in the plugin so file and introduces new macro:

VLIB_PLUGIN_REGISTER () = {
  .version = "version string",
  .version_required = "requred version",
  .default_disabled = 1,
  .early_init = "early_init_function_name",
};

Plugin will nor be loaded if .default_disabled is set to 1
unless explicitely enabled in startup.conf.

If .verstion_required is set, plugin will not be loaded if there
is version mismatch between plugin and vpp. This can be bypassed
by setting "skip-version-check" for specific plugin.

If .early-init string is present, plugin loader will try to resolve
this specific symbol in the plugin namespace and make a function call.

Following startup.conf configuration is added:

plugins {
  path /path/to/plugin/directory
  plugin ila_plugin.so { enable skip-version-check }
  plugin acl_plugin.so { disable }
}

Change-Id: I706c691dd34d94ffe9e02b59831af8859a95f061
Signed-off-by: Damjan Marion <damarion@cisco.com>