lib: hard-fail creating threads before fork()

lib: hard-fail creating threads before fork()

Creating any threads before we fork() into the background (if `-d` is
given) is an extremely dangerous footgun;  the threads are created in
the parent and terminated when that exits.

This is extra dangerous because while testing, you'd often run the
daemon in foreground without `-d`, and everything works as expected.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>