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, 90 insertions, 2 deletions

diff --git a/regress/mdoc/Fo/Makefile b/regress/mdoc/Fo/Makefile
index 7770e9a1..ce25ddf3 100644
--- a/regress/mdoc/Fo/Makefile
+++ b/regress/mdoc/Fo/Makefile
@@ -1,8 +1,10 @@
-# $OpenBSD: Makefile,v 1.17 2020/02/27 01:25:58 schwarze Exp $
+# $OpenBSD: Makefile,v 1.18 2020/03/13 00:31:06 schwarze Exp $
 
 REGRESS_TARGETS	 = basic break eos font noarg nohead
-REGRESS_TARGETS += obsolete punct section transp warn
+REGRESS_TARGETS += obsolete punct section tag transp warn
+TAG_TARGETS	 = tag
 LINT_TARGETS	 = noarg nohead obsolete punct warn
+HTML_TARGETS	 = tag
 
 # groff-1.22.3 defects:
 # - .Fo without an argument prints unbalanced parentheses
diff --git a/regress/mdoc/Fo/tag.in b/regress/mdoc/Fo/tag.in
new file mode 100644
index 00000000..727fdaf1
--- /dev/null
+++ b/regress/mdoc/Fo/tag.in
@@ -0,0 +1,29 @@
+.\" $OpenBSD: tag.in,v 1.1 2020/03/13 00:31:06 schwarze Exp $
+.Dd $Mdocdate$
+.Dt FO-TAG 1
+.Os
+.Sh NAME
+.Nm Fo-tag
+.Nd tagging of function name macros
+.Sh DESCRIPTION
+BEGINTEST
+.Pp
+automatic:
+.Fn first
+and
+.Fn second
+.Pp
+.Fn second
+and
+.Fn first
+.Pp
+explicit:
+.Tg e3
+.Fn third
+and
+.Tg e4
+.Fo fourth
+.Fa void
+.Fc
+.Pp
+ENDTEST
diff --git a/regress/mdoc/Fo/tag.out_ascii b/regress/mdoc/Fo/tag.out_ascii
new file mode 100644
index 00000000..505c0b7a
--- /dev/null
+++ b/regress/mdoc/Fo/tag.out_ascii
@@ -0,0 +1,17 @@
+FO-TAG(1)                   General Commands Manual                  FO-TAG(1)
+
+NNAAMMEE
+     FFoo--ttaagg - tagging of function name macros
+
+DDEESSCCRRIIPPTTIIOONN
+     BEGINTEST
+
+     automatic: ffiirrsstt() and sseeccoonndd()
+
+     sseeccoonndd() and ffiirrsstt()
+
+     explicit: tthhiirrdd() and ffoouurrtthh(_v_o_i_d)
+
+     ENDTEST
+
+OpenBSD                         March 13, 2020                         OpenBSD
diff --git a/regress/mdoc/Fo/tag.out_html b/regress/mdoc/Fo/tag.out_html
new file mode 100644
index 00000000..b3508614
--- /dev/null
+++ b/regress/mdoc/Fo/tag.out_html
@@ -0,0 +1,9 @@
+<p class="Pp">automatic:
+    <a class="permalink" href="#first"><code class="Fn" id="first">first</code></a>()
+    and <code class="Fn">second</code>()</p>
+<p class="Pp"><a class="permalink" href="#second"><code class="Fn" id="second">second</code></a>()
+    and <code class="Fn">first</code>()</p>
+<p class="Pp">explicit:
+    <a class="permalink" href="#e3"><code class="Fn" id="e3">third</code></a>()
+    and
+    <a class="permalink" href="#e4"><code class="Fn" id="e4">fourth</code></a>(<var class="Fa">void</var>);</p>
diff --git a/regress/mdoc/Fo/tag.out_markdown b/regress/mdoc/Fo/tag.out_markdown
new file mode 100644
index 00000000..273a00d4
--- /dev/null
+++ b/regress/mdoc/Fo/tag.out_markdown
@@ -0,0 +1,27 @@
+FO-TAG(1) - General Commands Manual
+
+# NAME
+
+**Fo-tag** - tagging of function name macros
+
+# DESCRIPTION
+
+BEGINTEST
+
+automatic:
+**first**()
+and
+**second**()
+
+**second**()
+and
+**first**()
+
+explicit:
+**third**()
+and
+**fourth**(*void*)
+
+ENDTEST
+
+OpenBSD - March 13, 2020
diff --git a/regress/mdoc/Fo/tag.out_tag b/regress/mdoc/Fo/tag.out_tag
new file mode 100644
index 00000000..2387023c
--- /dev/null
+++ b/regress/mdoc/Fo/tag.out_tag
@@ -0,0 +1,4 @@
+first 9
+second 11
+e3 13
+e4 13