ox-latex: Use \ul (from soul package) instead of \underline

* lisp/org.el (org-latex-default-packages-alist): Document need for
  "soul" package.
* lisp/ox-latex.el (org-latex-text-markup-alist): Use \ul (from "soul"
  package) instead of \underline for underline text.

lisp/org.el

@@ -3828,6 +3828,7 @@ Org mode to function properly:
   symbols used for interpreting the entities in `org-entities'.
   You can skip some of these packages if you don't use any of the
   symbols in it.
+- soul: for underline and strike-through
 - graphicx: for including images
 - float, wrapfig: for figure placement
 - longtable: for long tables

lisp/ox-latex.el

@@ -525,7 +525,7 @@ When nil, no transformation is made."
 					 (code . verb)
 					 (italic . "\\emph{%s}")
 					 (strike-through . "\\st{%s}")
-					 (underline . "\\underline{%s}")
+					 (underline . "\\ul{%s}")
 					 (verbatim . protectedtexttt))
   "Alist of LaTeX expressions to convert text markup.