Bring the handling of defective prologues even closer to groff,

Bring the handling of defective prologues even closer to groff,
in particular relaxing the distinction between prologue and body
and further improving messages.
* The last .Dd wins and the last .Os wins, even in the body.
* The last .Dt before the first body macro wins.
* Missing title in .Dt defaults to UNTITLED.  Warn about it.
* Missing section in .Dt does not default to 1.  But warn about it.
* Do not warn multiple times about the same mdoc(7) prologue macro.
* Warn about missing .Os.
* Incomplete .TH defaults to empty strings.  Warn about it.