feat: HTTP/1.1 chunked transfer encoding

[mgrossmann]

Summary

• Upgrades response protocol from HTTP/1.0 to HTTP/1.1
• Implements RFC 7230 chunked transfer encoding for responses without known Content-Length
• Static UFS files get Content-Length via ufs_stat(), SSI-processed files use chunked
• Validates Host header (400 Bad Request if missing on HTTP/1.1 requests)
• Adds Connection: close on all responses (keep-alive is a separate phase)
• Response version mirrors client version (HTTP/1.0 clients get HTTP/1.0 response)
• CGI modules (mvsMF) are unaffected — they already set Content-Length

Changed files

File	Change
include/httpd.h	New HTTPC fields: chunked (0x41), content_length_set (0x52)
include/httpcgi.h	Mirror HTTPC layout changes for CGI headers
src/httpresp.c	HTTP/1.1 response line (or 1.0 for 1.0 clients), Connection: close
src/httpsend.c	Chunk framing via send_raw() helper when chunked==1
src/httpdone.c	Send terminating chunk 0\r\n\r\n before CSTATE_REPORT
src/httpget.c	Content-Length for non-SSI UFS files, chunked for SSI (HTTP/1.1 only)
src/httppars.c	SERVER_PROTOCOL → HTTP/1.1
src/httpin.c	Host header validation for HTTP/1.1
src/httprese.c	Reset chunked and content_length_set between requests

Test results

• HTTP/1.1 response line
• HTTP/1.0 client → HTTP/1.0 response, no chunked
• Chunked for HTML/SSI files (size unknown at open time)
• Content-Length for static non-SSI files (e.g. favicon: 32038 bytes)
• mvsMF JSON endpoints → Content-Length (set by mvsMF)
• Missing Host header → 400 Bad Request
• Binary file download with Content-Length
• 404 for missing files
• Zowe CLI — no regressions

Fixes #50