Instead of panicking on a return(synth()) in vcl_pipe, we just proceed
to a normal synth.

Test by Dridi

Fixes #1890

eat all CPU time.

in both client and backend context.

Connections to backends so configured are never reused.

at client/server IPs.

what sockaddr-hack it is destined for.

the code with them.

for the lack of any better place to put them right now.

the central code, rather than the completion function.

It was decided during VDD15Q3 but this change didn't make it before the
4.1.0 release. It is kept in the 4.1 series but documented as deprecated
and can now be retired as discussed during VDD15Q4.

With a minor tweak so that running `make lcov` is enough, rather than
the original `make check-code-coverage`:

   ./autogen.sh
   ./configure --enable-code-coverage
   make lcov

Closes #1891

Most of it is rescued from the old Trac wiki.

I shouldn't rely on auto-complete too much, especially for only two
characters...

Spotted by @fgsch

It also applies for the 4.1 and 4.0 series.

Refs #1890

Fixes #1889.