ldpd: detach stdin/stdout/stderr from the child processes

ldpd: detach stdin/stdout/stderr from the child processes

Doing a "ssh user@node 'ldpd -d'" was making the SSH session hang. In
the original OpenBSD's ldpd(8) daemon, the daemon function takes care
of connecting stdin/stdout/stderr to /dev/null. In the FRR port, this
only happens in the frr_run() function, after all children have been
forked. Ideally we could try to rearrange libfrr.c and ldpd.c in a way
that start_child() is called only after the parent connects the standard
I/O streams to /dev/null. But since this issue needs an immediate
fix, let's do this workaround for now. Note: even when running on the
foreground, all log messages from the child processes are sent to the
parent process, which then prints the messages to stdout/stderr and/or
to a log file.

Reported-by: Martin Winter <mwinter@opensourcerouting.org>
Signed-off-by: Renato Westphal <renato@opensourcerouting.org>