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.

new regression tests for mdoc_macro.c revs. 1.167-1.172

When a full block macro gets closed out by a mismatching
block closure macro it calls, do not attempt to open its body.
This can for example happen for (nonsensical) constructions like
.Fo
.Nm Fc
in the SYNOPSIS. Fixing an assertion failure jsg@ found with afl
some time ago (test case number 731).