As VRT_Healthy needs a VRT_CTX, we move vcl_get_ctx and vcl_rel_ctx
to cache_varnishd.h scope as VCL_Get_CliCtx and VCL_Rel_CliCtx.

It has been discussed before if all varnishd cli commands should have
a VRT_CTX, so the natural solution would seem  to add one in or close
to VCLS_Poll(), yet CLI commands need specific ctxes (e.g. vcl.load
needs VCL_MET_INIT), so (for now) it appears sensible to limit the
ctx' scope to individual backend method calls.

Fixes #2887