adapt to new <p> output logic (html.c rev. 1.131)

Represent mdoc(7) .Pp (and .sp, and some SYNOPSIS and .Rs features)
by the <p> HTML element and use the html_fillmode() mechanism
for .Bd -unfilled, just like it was done for man(7) earlier, finally

Represent mdoc(7) .Pp (and .sp, and some SYNOPSIS and .Rs features)
by the <p> HTML element and use the html_fillmode() mechanism
for .Bd -unfilled, just like it was done for man(7) earlier, finally
getting rid both of the horrible <div class="Pp"></div> hack and
of the worst HTML syntax violations caused by nested displays.

Care is needed because in some situations, paragraphs have to remain
open across several subsequent macros, whereas in other situations,
they must get closed together with a block containing them.

Some implementation details include:
* Always close paragraphs before emitting HTML flow content.
* Let html_close_paragraph() also close <pre> for extra safety.
* Drop the old, now unused function print_paragraph().
* Minor adjustments in the top-level man(7) node formatter for symmetry.
* Bugfix: .Ss heads suspend no-fill mode, even though .Ss doesn't end it.
* Bugfix: give up on .Op semantic markup for now, see the comment.

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adapt to hex format of character entities,
committed by bentley@ in html.c rev. 1.86

Make HTML output more human readable by overhauling line break logic
around tags and by introducing some simple indentation.
No change of HTML semantics intended.

test various recent improvements of special character rendering