ldpd: add synchronous IPC channels

ldpd: add synchronous IPC channels

By default all ldpd interprocess communication is asynchronous
(non-blocking socketpairs). Under some circumstances, however, we'll
need synchronous IPC as well. Examples:
* the lde child process requesting labels to zebra (through the parent
  process);
* apply an access-list on a given IP prefix (ACLs are only available in
  the parent process).

This patch only adds the necessary infrastructure to allow the child
processes to make synchronous requests to the parent process. Later
patches will make use of this new infrastructure.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>