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.

6 files changed, 84 insertions, 2 deletions

diff --git a/regress/mdoc/Sy/Makefile b/regress/mdoc/Sy/Makefile
index 36c1859b..752d1382 100644
--- a/regress/mdoc/Sy/Makefile
+++ b/regress/mdoc/Sy/Makefile
@@ -1,6 +1,8 @@
-# $OpenBSD: Makefile,v 1.5 2014/11/17 06:44:35 schwarze Exp $
+# $OpenBSD: Makefile,v 1.8 2020/03/13 00:31:06 schwarze Exp $
 
-REGRESS_TARGETS = noarg font punct
+REGRESS_TARGETS = noarg font punct tag
+TAG_TARGETS	= tag
 LINT_TARGETS	= noarg punct
+HTML_TARGETS	= tag
 
 .include <bsd.regress.mk>
diff --git a/regress/mdoc/Sy/tag.in b/regress/mdoc/Sy/tag.in
new file mode 100644
index 00000000..b2719093
--- /dev/null
+++ b/regress/mdoc/Sy/tag.in
@@ -0,0 +1,23 @@
+.\" $OpenBSD: tag.in,v 1.1 2020/03/13 00:31:06 schwarze Exp $
+.Dd $Mdocdate$
+.Dt SY-TAG 1
+.Os
+.Sh NAME
+.Nm Sy-tag
+.Nd tagging of symbolic font macros
+.Sh DESCRIPTION
+BEGINTEST
+.Bl -tag -width Ds
+.It Sy one | two
+text
+.It Xo
+.Sy three
+.Xc
+text
+.El
+.Sy four
+.Sy one
+.Tg explicit
+.Sy five
+.Pp
+ENDTEST
diff --git a/regress/mdoc/Sy/tag.out_ascii b/regress/mdoc/Sy/tag.out_ascii
new file mode 100644
index 00000000..3b89cb41
--- /dev/null
+++ b/regress/mdoc/Sy/tag.out_ascii
@@ -0,0 +1,17 @@
+SY-TAG(1)                   General Commands Manual                  SY-TAG(1)
+
+NNAAMMEE
+     SSyy--ttaagg - tagging of symbolic font macros
+
+DDEESSCCRRIIPPTTIIOONN
+     BEGINTEST
+
+     oonnee | ttwwoo
+             text
+
+     tthhrreeee   text
+     ffoouurr oonnee ffiivvee
+
+     ENDTEST
+
+OpenBSD                         March 13, 2020                         OpenBSD
diff --git a/regress/mdoc/Sy/tag.out_html b/regress/mdoc/Sy/tag.out_html
new file mode 100644
index 00000000..69276a18
--- /dev/null
+++ b/regress/mdoc/Sy/tag.out_html
@@ -0,0 +1,10 @@
+<dl class="Bl-tag">
+  <dt><a class="permalink" href="#one"><b class="Sy" id="one">one</b></a> |
+    <a class="permalink" href="#two"><b class="Sy" id="two">two</b></a></dt>
+  <dd>text</dd>
+  <dt><a class="permalink" href="#three"><b class="Sy" id="three">three</b></a></dt>
+  <dd>text</dd>
+</dl>
+<a class="permalink" href="#four"><b class="Sy" id="four">four</b></a>
+  <b class="Sy">one</b>
+  <a class="permalink" href="#explicit"><b class="Sy" id="explicit">five</b></a>
diff --git a/regress/mdoc/Sy/tag.out_markdown b/regress/mdoc/Sy/tag.out_markdown
new file mode 100644
index 00000000..5382a5e2
--- /dev/null
+++ b/regress/mdoc/Sy/tag.out_markdown
@@ -0,0 +1,25 @@
+SY-TAG(1) - General Commands Manual
+
+# NAME
+
+**Sy-tag** - tagging of symbolic font macros
+
+# DESCRIPTION
+
+BEGINTEST
+
+**one** | **two**
+
+> text
+
+**three**
+
+> text
+
+**four**
+**one**
+**five**
+
+ENDTEST
+
+OpenBSD - March 13, 2020
diff --git a/regress/mdoc/Sy/tag.out_tag b/regress/mdoc/Sy/tag.out_tag
new file mode 100644
index 00000000..c2fbaf59
--- /dev/null
+++ b/regress/mdoc/Sy/tag.out_tag
@@ -0,0 +1,5 @@
+one 9
+two 9
+three 12
+four 13
+explicit 13