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.

(cherry picked from commit 1cdceb936521ca4dc06d4f57af909310ec374a5e)
Signed-off-by: Willy Tarreau <w@1wt.eu>