ldpd: simplify initialization of the child processes

ldpd: simplify initialization of the child processes

In order to have separate ASLR/cookies per process, ldpd calls exec()
in the child processes after fork() (this is also known as the fork+exec
model).

This is an important security feature but it makes the initialization
of the child processes a bit more complicated as they're not a copy of
the parent anymore, so all parameters given via command line are lost.

To solve this problem, we were creating an argv array by hand with all
necessary parameters and providing it to the exec() syscall. This works
but it's a very ugly solution. This patch introduces a different approach
to solve the problem: send an IMSG_INIT message to the child processes
with all parameters they need in order to initialize properly. This
makes adding additional initialization parameters much more convenient
and less error prone.

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