api: Server core

This patch adds the server core of an in-tree JSON/HTTP API server to
OSv. It uses epoll() for sockets, HTTP parser library by Joyent, and
JSON library by Dropbox.

Sources for HTTP parser by Joyent taken from:

  commit 8d9e5db981b623fffc93657abacdc80270cbee58

  https://github.com/joyent/http-parser

Sources for JSON library by Dropbox taken from::

  commit 80578664de72b23992f9ac08b17ae9d336dc1fa9

  https://github.com/dropbox/json11

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>