MINOR: htx: Add a function to return a block at a specific offset

MINOR: htx: Add a function to return a block at a specific offset

The htx_find_offset() function may be used to look for a block at a specific
offset in an HTX message, starting from the message head. A compound result is
returned, an htx_ret structure, with the found block and the position of the
offset in the block. If the offset is ouside of the HTX message, the returned
block is NULL.