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.

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