feat: automatic chunked transfer encoding for CGI responses

[mgrossmann]

Summary

• Auto-inject Transfer-Encoding: chunked in httpprtv.c when the header-ending blank line is sent without Content-Length or chunked for HTTP/1.1 clients
• Auto-detect Content-Length: headers from any caller (including external CGI modules like mvsMF)
• Remove explicit chunked logic from httpget.c — now handled centrally
• Reset rdw flag in httprese.c for keep-alive request cycling

How it works

httpprtv.c is the central printf path all code uses (HTTPD core + CGI modules via HTTPX vector). When it sees:

1. A Content-Length: header → sets content_length_set flag
2. The header-ending blank line ("\r\n", 2 bytes) → checks if chunked fallback is needed
3. If HTTP/1.1 and no Content-Length/chunked → injects Transfer-Encoding: chunked header, enables chunk framing

Test plan

• Build on MVS (make build && make link)
• CGI pages (HTTPDSRV, HTTPDM, HTTPDMTT) show Transfer-Encoding: chunked
• Browser renders CGI pages immediately, no timeout waiting
• mvsMF API responses still use Content-Length (not chunked)
• Static files with known size still use Content-Length
• Static files without known size (SSI) use chunked
• HTTP/1.0 clients receive no chunked encoding
• Keep-alive works correctly across multiple requests

Fixes #58