HTTP/2, header conversion tightening

[icing]

• fold the code to convert dynhds to the nghttp2 structs into a dynhds internal method
• saves code duplication
• pacifies compiler analyzers

[vszakats]

Warnings reduced to a single block after the patch.

Before:
https://github.com/curl/curl-for-win/actions/runs/6499470079/job/17652877625#step:3:5316

After:
https://github.com/curl/curl-for-win/actions/runs/6499706712/job/17655051866#step:3:5324

/Users/runner/work/curl-for-win/curl-for-win/curl/lib/vquic/curl_ngtcp2.c:1709:38: warning: potential null pointer dereference [-Wnull-dereference]
 1709 |     nva[i].value = (unsigned char *)e->value;
      |                                     ~^~~~~~~
/Users/runner/work/curl-for-win/curl-for-win/curl/lib/vquic/curl_ngtcp2.c:1708:23: warning: potential null pointer dereference [-Wnull-dereference]
 1708 |     nva[i].namelen = e->namelen;
      |                      ~^~~~~~~~~
/Users/runner/work/curl-for-win/curl-for-win/curl/lib/vquic/curl_ngtcp2.c:1707:37: warning: potential null pointer dereference [-Wnull-dereference]
 1707 |     nva[i].name = (unsigned char *)e->name;
      |                                    ~^~~~~~
/Users/runner/work/curl-for-win/curl-for-win/curl/lib/vquic/curl_ngtcp2.c:1710:24: warning: potential null pointer dereference [-Wnull-dereference]
 1710 |     nva[i].valuelen = e->valuelen;
      |                       ~^~~~~~~~~~

[jay]

That's also true of msh3 and quiche. I think the pattern could be eliminated from all of vquic. Otherwise it looks fine.