MINOR: htx: add function to set EOM reliably

MINOR: htx: add function to set EOM reliably

Implement a new HTX utility function htx_set_eom(). If the HTX message
is empty, it will first add a dummy EOT block. This is a small trick
needed to ensure readers will detect the HTX buffer as not empty and
retrieve the EOM flag.

Replace the H2 code related by a htx_set_eom() invocation. QUIC also has
the same code which will be replaced in the next commit.

This should be backported up to 2.7 before the related QUIC patch.