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.

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

Multiple fixes with respect to in-line macros:
* .No selects the default font; relevant e.g. in .Bf blocks
* no need to force empty .Li elements
* closing delimiters as leading macro arguments do not suppress space
* opening delimiters at the end of a macro line do not suppress space
* correctly handle delimiter spacing in -Tman
As a side effect, these fixes let mandoc warn about empty .No macros
as requested by bentley@.

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

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

Multiple fixes with respect to in-line macros:
* .No selects the default font; relevant e.g. in .Bf blocks
* no need to force empty .Li elements
* closing delimiters as leading macro arguments do not suppress space
* opening delimiters at the end of a macro line do not suppress space
* correctly handle delimiter spacing in -Tman
As a side effect, these fixes let mandoc warn about empty .No macros
as requested by bentley@.

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