MINOR: htx: Add a function to know if a block is the only one in a message

MINOR: htx: Add a function to know if a block is the only one in a message

The htx_is_unique_blk() function may now be used to know if a block is the
only one in an HTX message, excluding all unused blocks. Note the purpose of
this function is not to know if a block is the last one of an HTTP message.
This means no more data part from the message are expected, except tunneled
data. It only says if a block is alone in an HTX message.