MAJOR: htx: Use htx_set_eom() instead of setting HTX_FL_EOM by hand

MAJOR: htx: Use htx_set_eom() instead of setting HTX_FL_EOM by hand

The function htx_set_eom() takes care to the HTX_FL_EOM on the HTX message,
by first adding a EOT block if the message is empty. So let's use it
wherever possible.

This commit is tagged as MAJOR because it is an API change.