MEDIUM: htx: prevent <mark> to copy incomplete headers in htx_xfer_blks()

MEDIUM: htx: prevent <mark> to copy incomplete headers in htx_xfer_blks()

Prevent a partial copy of trailers or headers when using the <mark>
parameter.

When using htx_xfer_blks(), transfering partial headers or trailers are
prevented when restricted by the <count> parameter. However using the
<mark> parameter will still allow to do it.

This patch changes the behavior by checking the <mark> type only after
checking the headers/trailers type, so we can still rollback on partial
transfer.

No impact on the current code, which does not try to do that yet.