MEDIUM: htx: Skip UNUSED blocs when getting previous,next and first blocks

MEDIUM: htx: Skip UNUSED blocs when getting previous,next and first blocks

htx_get_prev_blk(), htx_get_next_blk() and htx_get_first_blk() functions
were modified to automatically skip UNUSED blocks. So now, a valid block or
NULL is returned.  In addition, htx_remove_blk() was adapted accordingly to
also skip UNUSED blocks.

Thanks to this change, the loops on HTX messages could be simplified by
removing test on UNUSED blocks, while it remains harmless.