proc: expose forwarded headers to CGI

[Antecer]

Expose common reverse proxy headers to CGI environments.

uhttpd currently maps only a fixed set of request headers into CGI
environment variables. When LuCI or CGI applications are served behind a
reverse proxy, standard forwarding headers such as X-Forwarded-For and
X-Real-IP are not visible to backend scripts.

This prevents applications from determining the original client address
when uhttpd is behind a trusted reverse proxy. Expose the common forwarded
headers as HTTP_* CGI variables, matching the existing header whitelist
mechanism.

Added:

• HTTP_X_FORWARDED_FOR
• HTTP_X_FORWARDED_HOST
• HTTP_X_FORWARDED_PORT
• HTTP_X_FORWARDED_PROTO
• HTTP_X_REAL_IP
• HTTP_X_REAL_PROTO

Tested:

• git diff --check
• Build not run locally because cmake is not available in this environment.