Add support for inet diag Netlink protocol.

- Inet diag allows users to gather low-level socket information.
- This library provides a higher-level API for creating inetdiag requests (via
  idiagnl_connect and idiagnl_send_simple) and parsing the replies (via
  idiagnl_msg_parse). A cache is also provided (via idiagnl_msg_alloc_cache).
- Request and message objects provide APIs for accessing and setting the
  various properties of each.
- This library also allows the user to parse the inetdiag response attributes
  which contain information about traffic class, TOS, congestion, socket
  memory info, and more depending on the kernel version used.
- Includes doxygen documentation.
Handle the case where nl_msec2str is passed 0 msecs