Split tagging into a validation part including prioritization

in tag.{h,c} and {mdoc,man}_validate.c
and into a formatting part including command line argument checking
in term_tag.{h,c}, html.c, and {mdoc|man}_{term|html}.c.

Immediate functional benefits include:
* Improved prioritization of automatic tags for .Em and .Sy.
* Avoiding bogus automatic tags when .Em, .Fn, or .Sy are explicitly tagged.
* Explicit tagging of .Er and .Fl now works in HTML output.
* Automatic tagging of .IP and .TP now works in HTML output.
But mainly, this patch provides clean earth to build further improvements on.

Technical changes:
* Main program: Write a tag file for ASCII and UTF-8 output only.
* All formatters: There is no more need to delay writing the tags.
* mdoc(7)+man(7) formatters: No more need for elaborate syntax tree inspection.
* HTML formatter: If available, use the "string" attribute as the tag.
* HTML formatter: New function to write permalinks, to reduce code duplication.

Style cleanup in the vicinity while here:
* mdoc(7) terminal formatter: To set up bold font for children,
defer to termp_bold_pre() rather than calling term_fontpush() manually.
* mdoc(7) terminal formatter: Garbage collect some duplicate functions.
* mdoc(7) HTML formatter: Unify <code> handling, delete redundant functions.
* Where possible, use switch statements rather than if cascades.
* Get rid of some more Yoda notation.

The necessity for such changes was first discussed with kn@, but i didn't
bother him with a request to review the resulting -673/+782 line patch.

1 files changed, 34 insertions, 0 deletions

diff --git a/regress/mdoc/Tg/warn.in b/regress/mdoc/Tg/warn.in
new file mode 100644
index 00000000..e36f4506
--- /dev/null
+++ b/regress/mdoc/Tg/warn.in
@@ -0,0 +1,34 @@
+.\" $OpenBSD: warn.in,v 1.1 2020/03/13 00:31:06 schwarze Exp $
+.Dd $Mdocdate: March 13 2020 $
+.Dt TG-WARN 1
+.Os
+.Sh NAME
+.Nm Tg-warn
+.Nd warnings about tagging macros
+.Sh DESCRIPTION
+BEGINTEST
+.Pp
+.Tg start
+initial
+text
+.Tg
+.Ic macro
+.Tg "" ignored arguments
+too many
+.Tg \&badstart
+badstart
+.Tg badend\&
+badend
+.Tg "white space"
+whitespace
+.Tg sub
+.Tg double
+.Ss Subsection
+subtext
+.Tg examples
+.Sh EXAMPLES
+example
+text
+.Pp
+ENDTEST
+.Tg