info-look fixes for Texinfo 5

info-look fixes for Texinfo 5

* info-look.el (c-mode, bison-mode, makefile-mode)
(makefile-automake-mode, texinfo-mode, autoconf-mode, awk-mode)
(latex-mode, emacs-lisp-mode, sh-mode, cfengine-mode): Match
`foo' and 'foo' and ‘foo’ for @item and similar.
(latex-mode): Match multi-arg \frac{num}{den} or \sqrt[root]{n} in
suffix regexp.