MEDIUM: htx: Insert headers and trailers before corresponding end-of block

MEDIUM: htx: Insert headers and trailers before corresponding end-of block

htx_add_header() and htx_add_trailer() were updated to automatically insert
the block at the right place, just before the corresponding end-of block.
Indeed, it is invalid, unexepected and probably unsupported to insert a
header after the EOH block without moving it. So it is easier to handle it
in HTX function directly.

It means that http_add_header() was simplified.