Fix <p> and <div> newlines with or without <li> in shr

Fix <p> and <div> newlines with or without <li> in shr

* lisp/net/shr.el (shr-ensure-newline): Respect that we're in
a <li>, if we are, and don't insert newlines there.
(shr-ensure-paragraph): When mixing newlines and paragraph
ensurements, don't insert too many blank lines.
(shr-tag-div): A <div> shouldn't introduce a paragraph, but a
new line.