CLI to control backend health, rather than rely on probes.

Submitted by:	Federico G. Schwindt

The attached patch is a one-liner for waiter/cache_waiter_ports.c,
the code was referencing sp->t_open where it should have been
sp->t_idle, which was causing assertion failures on idle session
timeouts.

I work with NetBeans, and it insists on this being present.

if sub is null (because it's the contents of a non-existing header),
pretend it's "" instead.

Add a link to the varnishlog man page and the logging chapter in the tutorial

remove -i and -I, add -m and describe it. Simplify the docs. The user only needs to care about -m, really

I'm not sure if we should go into more detail about how the whole thing works

Fixes #1062