Until now, the VCC SUB type represented the literal name of a
vcc-generated C function, which was used to expand the VCL "call"
action.

We now add a struct to describe VCL subs and calculate a bitmask which
represents the bilt-in subs which a SUB may be called from (the
"okmask"),

This is in preparation of the goal to add a VCL_SUB vmod type which
will allow VMODs to call into vcl subs.

We add to the vcl shared object a struct vcl_sub for each sub, which
contains:

	- a methods bitmask defining which built-in subs this sub may
	  be called from (directly or indirectly)
	- the name as seen from vcl
	- a pointer back to the VCL_conf
	- the function pointer to the vcl_func_f
	- a unique number

About the methods bitmask:

It contains the VCL_MET_* bits of of built-in subs (traditionally
called methods) which are allowed to call this sub.

It is intended for checks like

	if (sub->methods & ctx->method) {
		sub->func(ctx);
		return;
	} else {
		VRT_fail(ctx, "not allowed here");
		return;
	}

In existing compile-time calls, we already check if used objects and
returned actions are allowed within the respective calling context.

To build the methods bitmask, we begin with a VCL_MET_TASK_ALL
bitfield and clear the bits of methods which would not be allowed for
any used object or returned action.

About the VCL_conf pointer:

Each VCL SUB belongs to a VCL, this pointer will allow to check that
it is only ever used from the right VCL.

About the unique number:

By numbering vcl subs (per VCL), we can later implement a recursion
check using a bitmask.

In struct VCL_conf, we record the total number of subs.