MINOR: htx: Add support for flags on HTX blocks

MINOR: htx: Add support for flags on HTX blocks

In the commit 32b51cdf3 ("BUG/MINOR: config: reject configs using HTTP with
bufsize >= 256 MB"), the maximum possible size for buffers is limited to 256
MB. So, on HTX side, it means a bloc cannot have a relative address higher
than this value. So on the 32-bits integer used to store the relative block
address, only 28 bits can be used, and 4 bits remains unused.

So these 4 bits were reused to store flags on block themselves. In this
patch, the support for these flags is introduced and the HTX API was adapted
to deal with it.