misc: move ppp to a plugin

misc: move ppp to a plugin

Type: refactor

Move PPP folder under vnet to the plugin folder, and modify some of path
of the #inlude<header> to the new path.

Add a plugin.c file to register a plugin.

Resolve ip4_input and ip6_input's dependency on PPP functions by moving
those calls to PPP's initialization.

Resolve osi's inter-plugin dependency on PPP by having it retrieve the
function pointer

Add ppp to the list of valid spelling words

JIRA: VPP-2052

Change-Id: I1a26ef0663a91857d13f7d87a3bb14bc38893194
Signed-off-by: Joel Ahn <[email protected]>