Just like the file cursor reads from stdin when opening "-". And in
addition, teach varnishlog to also write to stdout when it is opening
the file directly. Likewise, put varnishncsa on par with varnishlog in
that regard.

It is now possible to compress binary VSL on the fly without taking
a detour to the file system, see u00019.vtc for an example.

Considering how VUT daemonizes, a "-w -" option in daemon mode becomes
an error as well.

Refs #3740