that is,

	call vmod.returning_sub();

in VCL.

Background:

The existing

	call sub;

is special in that it instantiates "sub", if it does not exist
already, enabling use-before-definition semantics.

The doctrine of this change is to preserve this behaviour and to not
make existing, static calls any less efficient.

Implementation:

To support calling both literal SUBs as well as SUB expressions, we
peek into the symbol table to check if the called expression is a
function, based on the knowledge that, as of now, only functions can
have a non-literal SUB return. We also know that no other expression
can yield a SUB. So if peeking ahead tells us that a function follows,
we call into expression evaluation.

Otherwise, we expect a literal SUB as before.

Null check:

For dynamic SUB calls from vmods via VRT_call(), we require the SUB
argument be non-NULL. But we can not for call: To signal error with a
SUB return, a vmod function needs to be allowed to return NULL,
otherwise it would need to have a dummy return value available for the
VRT_fail() case. So we need to check for NULL in VGC.

Alternatives considered:

- We could make expression evaluation return SUB also for literal SUB
symbols, turning all calls into sub->func(ctx, ...) C statements. I
tried this path and it resulted in a change of behaviour with
cc_command="clang -g -O2 ...": Where, previously, sub code was
inlined, it would now generate an explicit C function call always,
despite the fact that the C function is known at compile time (because
all named VCL_SUB structs are).

So this option was dismissed for violating the doctrine.

- The null check could go to VPI, via a VPI_call(), basically identical
to VRT_call(), but checking for NULL. This option was dismissed because
it would foreclose the requirement to allow for SUB arguments in the
future.

Acknowledgements:

reza mentioned the idea of call SUB at one point, and I can not remember
if I had it before or not, so when in doubt, it was his idea.

Dridi helped with ideas on the implementation and spotted a bug.