MEDIUM: htx: Add HTX_BLK_FL_EOM flag and set it on last block of the message

MEDIUM: htx: Add HTX_BLK_FL_EOM flag and set it on last block of the message

HTX_BLK_FL_EOM flag was introduced to be set on the last block of an HTX
message. It must be set on the tail block when the HTX_FL_HAS_EOM is set.
Now, only the HTX API must set these flags by hand. From outside of the HTX
API, htx_set_eom() function must be used.

A BUG_ON() was added to unsure the flag is added on valid blocks. It is possible
to add it on EOH, EOT and DATA blocks only.