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.

7 files changed, 79 insertions, 2 deletions

diff --git a/regress/mdoc/No/Makefile b/regress/mdoc/No/Makefile
index 3110bf2b..35aa4e1e 100644
--- a/regress/mdoc/No/Makefile
+++ b/regress/mdoc/No/Makefile
@@ -1,6 +1,8 @@
-# $OpenBSD: Makefile,v 1.4 2014/11/17 06:44:35 schwarze Exp $
+# $OpenBSD: Makefile,v 1.7 2020/03/13 00:31:06 schwarze Exp $
 
-REGRESS_TARGETS = punct spacing
+REGRESS_TARGETS = punct spacing tag
+TAG_TARGETS	= tag
 LINT_TARGETS	= punct
+HTML_TARGETS	= tag
 
 .include <bsd.regress.mk>
diff --git a/regress/mdoc/No/punct.out_lint b/regress/mdoc/No/punct.out_lint
index e046da6a..6a65c3b4 100644
--- a/regress/mdoc/No/punct.out_lint
+++ b/regress/mdoc/No/punct.out_lint
@@ -23,3 +23,4 @@ mandoc: punct.in:72:7: WARNING: skipping empty macro: No
 mandoc: punct.in:75:7: WARNING: skipping empty macro: No
 mandoc: punct.in:76:7: WARNING: skipping empty macro: No
 mandoc: punct.in:84:2: WARNING: skipping empty macro: No
+mandoc: punct.in:87:6: STYLE: no blank before trailing delimiter: No a.
diff --git a/regress/mdoc/No/tag.in b/regress/mdoc/No/tag.in
new file mode 100644
index 00000000..a9d4415f
--- /dev/null
+++ b/regress/mdoc/No/tag.in
@@ -0,0 +1,21 @@
+.\" $OpenBSD: tag.in,v 1.1 2020/03/13 00:31:06 schwarze Exp $
+.Dd $Mdocdate: March 13 2020 $
+.Dt NO-TAG 1
+.Os
+.Sh NAME
+.Nm No-tag
+.Nd tagging of normal font macros
+.Sh DESCRIPTION
+BEGINTEST
+.Bl -tag -width Ds
+.It No one | two
+text
+.It Xo
+.No three
+.Xc
+text
+.El
+.Tg
+.No four
+.Pp
+ENDTEST
diff --git a/regress/mdoc/No/tag.out_ascii b/regress/mdoc/No/tag.out_ascii
new file mode 100644
index 00000000..44481b02
--- /dev/null
+++ b/regress/mdoc/No/tag.out_ascii
@@ -0,0 +1,17 @@
+NO-TAG(1)                   General Commands Manual                  NO-TAG(1)
+
+NNAAMMEE
+     NNoo--ttaagg - tagging of normal font macros
+
+DDEESSCCRRIIPPTTIIOONN
+     BEGINTEST
+
+     one | two
+             text
+
+     three   text
+     four
+
+     ENDTEST
+
+OpenBSD                         March 13, 2020                         OpenBSD
diff --git a/regress/mdoc/No/tag.out_html b/regress/mdoc/No/tag.out_html
new file mode 100644
index 00000000..32696917
--- /dev/null
+++ b/regress/mdoc/No/tag.out_html
@@ -0,0 +1,9 @@
+<dl class="Bl-tag">
+  <dt><a class="permalink" href="#one"><span class="No" id="one">one</span></a>
+    |
+    <a class="permalink" href="#two"><span class="No" id="two">two</span></a></dt>
+  <dd>text</dd>
+  <dt><a class="permalink" href="#three"><span class="No" id="three">three</span></a></dt>
+  <dd>text</dd>
+</dl>
+<a class="permalink" href="#four"><span class="No" id="four">four</span></a>
diff --git a/regress/mdoc/No/tag.out_markdown b/regress/mdoc/No/tag.out_markdown
new file mode 100644
index 00000000..1590c010
--- /dev/null
+++ b/regress/mdoc/No/tag.out_markdown
@@ -0,0 +1,23 @@
+NO-TAG(1) - General Commands Manual
+
+# NAME
+
+**No-tag** - tagging of normal font macros
+
+# DESCRIPTION
+
+BEGINTEST
+
+one | two
+
+> text
+
+three
+
+> text
+
+four
+
+ENDTEST
+
+OpenBSD - March 13, 2020
diff --git a/regress/mdoc/No/tag.out_tag b/regress/mdoc/No/tag.out_tag
new file mode 100644
index 00000000..94f0cfb2
--- /dev/null
+++ b/regress/mdoc/No/tag.out_tag
@@ -0,0 +1,4 @@
+one 9
+two 9
+three 12
+four 13