We are going to reuse the stanza format for IPC in the plugin protocol,
but in that context we need stanzas to be self-closing. Currently they
almost are, but if the body is 0 modulo 48, there is no way to know if
the stanza is over after the last line.
Now, all stanzas have to end with a short line, even if empty.
No ciphertexts generated by age in the past are affected, but 3% of the
ciphertexts generated by rage will now stop working. They are still
supported by rage going forward. If it turns out to be a common issue,
we can add an exception.