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, 80 insertions, 2 deletions

diff --git a/regress/mdoc/Er/Makefile b/regress/mdoc/Er/Makefile
index 1be2a84a..55f6cc76 100644
--- a/regress/mdoc/Er/Makefile
+++ b/regress/mdoc/Er/Makefile
@@ -1,6 +1,8 @@
-# $OpenBSD: Makefile,v 1.4 2014/07/02 20:18:42 schwarze Exp $
+# $OpenBSD: Makefile,v 1.7 2020/03/13 00:31:05 schwarze Exp $
 
-REGRESS_TARGETS = noarg font
+REGRESS_TARGETS = noarg font tag
+TAG_TARGETS	= tag
 LINT_TARGETS	= noarg
+HTML_TARGETS	= tag
 
 .include <bsd.regress.mk>
diff --git a/regress/mdoc/Er/tag.in b/regress/mdoc/Er/tag.in
new file mode 100644
index 00000000..4227648b
--- /dev/null
+++ b/regress/mdoc/Er/tag.in
@@ -0,0 +1,21 @@
+.\" $OpenBSD: tag.in,v 1.1 2020/03/13 00:31:05 schwarze Exp $
+.Dd $Mdocdate: March 13 2020 $
+.Dt ER-TAG 1
+.Os
+.Sh NAME
+.Nm Er-tag
+.Nd tagging of error number macros
+.Sh DESCRIPTION
+BEGINTEST
+.Bl -tag -width Ds
+.It Er one
+text
+.El
+.Tg
+.Er two
+.Sh ERRORS
+.Bl -tag -width Er
+.It Bq Er ENOENT
+text
+.El
+ENDTEST
diff --git a/regress/mdoc/Er/tag.out_ascii b/regress/mdoc/Er/tag.out_ascii
new file mode 100644
index 00000000..62dbf9ca
--- /dev/null
+++ b/regress/mdoc/Er/tag.out_ascii
@@ -0,0 +1,16 @@
+ER-TAG(1)                   General Commands Manual                  ER-TAG(1)
+
+NNAAMMEE
+     EErr--ttaagg - tagging of error number macros
+
+DDEESSCCRRIIPPTTIIOONN
+     BEGINTEST
+
+     one     text
+     two
+
+EERRRROORRSS
+     [ENOENT]           text
+     ENDTEST
+
+OpenBSD                         March 13, 2020                         OpenBSD
diff --git a/regress/mdoc/Er/tag.out_html b/regress/mdoc/Er/tag.out_html
new file mode 100644
index 00000000..a2c2978f
--- /dev/null
+++ b/regress/mdoc/Er/tag.out_html
@@ -0,0 +1,12 @@
+<dl class="Bl-tag">
+  <dt><code class="Er">one</code></dt>
+  <dd>text</dd>
+</dl>
+<a class="permalink" href="#two"><code class="Er" id="two">two</code></a>
+</section>
+<section class="Sh">
+<h1 class="Sh" id="ERRORS"><a class="permalink" href="#ERRORS">ERRORS</a></h1>
+<dl class="Bl-tag">
+  <dt>[<a class="permalink" href="#ENOENT"><code class="Er" id="ENOENT">ENOENT</code></a>]</dt>
+  <dd>text</dd>
+</dl>
diff --git a/regress/mdoc/Er/tag.out_markdown b/regress/mdoc/Er/tag.out_markdown
new file mode 100644
index 00000000..d11eb4b9
--- /dev/null
+++ b/regress/mdoc/Er/tag.out_markdown
@@ -0,0 +1,25 @@
+ER-TAG(1) - General Commands Manual
+
+# NAME
+
+**Er-tag** - tagging of error number macros
+
+# DESCRIPTION
+
+BEGINTEST
+
+`one`
+
+> text
+
+`two`
+
+# ERRORS
+
+\[`ENOENT`]
+
+> text
+
+ENDTEST
+
+OpenBSD - March 13, 2020
diff --git a/regress/mdoc/Er/tag.out_tag b/regress/mdoc/Er/tag.out_tag
new file mode 100644
index 00000000..b00a6718
--- /dev/null
+++ b/regress/mdoc/Er/tag.out_tag
@@ -0,0 +1,2 @@
+two 10
+ENOENT 13