proc: inhibit chunked transfer encoding on existing TEs or known length

proc: inhibit chunked transfer encoding on existing TEs or known length

When a backend CGI, Lua or ucode handler produces a response which either
includes a Content-Length or a Transfer-Encoding header then disable the
uhttpd side chunked transfer encoding and assume the backend response to
be already properly encoded or length delimitted.

Fixes: https://github.com/openwrt/luci/issues/7655
Signed-off-by: Jo-Philipp Wich <[email protected]>