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.

Macro argument quoting does not prevent recognition of punctuation
and of called macros.

This bug affects almost all macros, and fixing it simplifies the
code. It is amazing that the bogus ARGS_QWORD feature got implemented
in the first place, and then carrier along for more than eight years
without anybody ever noticing that it was pointless.

Reported by Leah Neukirchen <leah at vuxu dot org>, found on Void Linux.

Now that markdown output is tested for almost everything, test all
input files in -T markdown output mode by default and only mark
those files with SKIP_MARKDOWN that are not to be tested.
Much easier to read, and almost minus 40 lines of Makefile code.

first batch of -T markdown tests

Improve and test the messages about empty macros,
in particular reporting the macro names involved.

fix -Tman font handling for:
.Ad .Ar .Cd .Cm .Dv .Em .Er .Ev .Fa .Fl .Fn .Fo .Ft
.Ic .In .Lk .Li .Ms .Mt .Nm .Pa .Sx .Sy .Tn .Va .Vt

after several improvements to -Tman went in,
enable lots of related regression tests

Macro argument quoting does not prevent recognition of punctuation
and of called macros.

This bug affects almost all macros, and fixing it simplifies the
code. It is amazing that the bogus ARGS_QWORD feature got implemented
in the first place, and then carrier along for more than eight years
without anybody ever noticing that it was pointless.

Reported by Leah Neukirchen <leah at vuxu dot org>, found on Void Linux.

Now that markdown output is tested for almost everything, test all
input files in -T markdown output mode by default and only mark
those files with SKIP_MARKDOWN that are not to be tested.
Much easier to read, and almost minus 40 lines of Makefile code.

first batch of -T markdown tests

Improve and test the messages about empty macros,
in particular reporting the macro names involved.

fix -Tman font handling for:
.Ad .Ar .Cd .Cm .Dv .Em .Er .Ev .Fa .Fl .Fn .Fo .Ft
.Ic .In .Lk .Li .Ms .Mt .Nm .Pa .Sx .Sy .Tn .Va .Vt

after several improvements to -Tman went in,
enable lots of related regression tests