Prefer matching line beginnings in org-export-latex-special-chars
* lisp/org-latex.el (org-export-latex-special-chars): Fix regexp for
`single' special characters and ellipsis.
Repeated special characters are exported differently depending on
their position in the buffer. A "&&" string at the start of a line is
exported as "&\&" whereas in the middle of a line you get "\&\&". The
former is incorrect. Fix this by matching the beginning of a line
before a character. While we're at it, amalgamate the regexps for the
different special characters.