Messages of the -Wbase level now print STYLE:. Since this
causes horrible churn anyway, profit of the opportunity to stop
excessive testing, such that this is hopefully the last instance
of such churn. Consistently use OpenBSD RCS tags, blank .Os,
blank fourth .TH argument, and Mdocdate like everywhere else.
Use -Ios=OpenBSD for platform-independent predictable output.

cope with changes in BASE messages

churn related to the new style message about RCS ids

churn caused by the new Mdocdate messages, no easy way to avoid this :(

improve NAME section diagnostics;
confusing messages reported by Jan Stary <hans at stare dot cz>

start catching up with recent code changes, but no new files yet

Fix the column numbers associated with in_line_argn() macros;
this bug is more than four years old, introduced by kristaps@
in mdocml.bsd.lv rev. 1.46, March 30, 2010.

Clean up the warnings related to document structure.
* Hierarchical naming of the related enum mandocerr items.
* Mention the offending macro, section title, or string.
While here, improve some wordings:
* Descriptive instead of imperative style.
* Uniform style for "missing" and "skipping".
* Where applicable, mention the fallback used.

regression tests for prologue warnings