MINOR: mux: add sctl MUX_SCTL_GET_HDNG_FOUND/FIXED to report hardening results

MINOR: mux: add sctl MUX_SCTL_GET_HDNG_FOUND/FIXED to report hardening results

The H1, H2 and H3 streams store in their http_hdng_chk what HTTP hardening
conditions were met (found) and what rules were enforced (fixed). This adds
two new mux stream controls to retrieve them as uint32_t, so that they can
be reported by sample fetches and logged:

  - MUX_SCTL_GET_HDNG_FOUND returns hdng_chk.found
  - MUX_SCTL_GET_HDNG_FIXED returns hdng_chk.fixed

They're implemented in the H1 and H2 muxes. The QUIC mux passes them to
the application layer's sctl() after checking that it's H3, and h3_sctl()
implements them. Other muxes return -1 as they don't support hardening.