Ref 43d9e5fb :

PRIV_* fini methods need to leave (struct vrt_ctx).handling alone,
except that they might call VRT_fail(), see also
746384b2

Thus we add assertions that handling be either 0 or VCL_RET_FAIL
outside the FSM.

To be able to do so, we need to change VCL_RET_OK into 0 when
vcl_init{} has returned successfully.

The vcl_fini{} case is slightly more complicated:

By design, only "ok" (VCL_RET_OK) is allowed, but VRT_fail() also
added VCL_RET_FAIL, so we de-facto get a "fail" return if any vmod
code called VRT_fail().

Because PRIV_* handling happens from VCC generated code via
VGC_Discard(), we need to check and change (struct vrt_ctx).handling
right after calling vcl_fini{} / VGC_function_vcl_fini() from
VGC_Discard(). This is VPI_vcl_fini().

Implementation note:

I also considered void VPI_vcl_fini(VRT_CTX, vcl_func_f fini_sub),
having VPI_vcl_fini call the fini_sub, but that stirred up includes of
VPI where vcl.h is not included.