(occur): Use text property `occur' to store the
marker for the occurrence in the source buffer. This replaces the
list `occur-pos-list', and fixes the bug for multi-line matches.
Set up `occur-point' text property for occur-next and occur-prev.
(occur): occur-num-matches stores the number of matches found.
(occur-mode-find-occurrence): Use `occur' text property to find
marker for locus of the occurrence.
(occur-next, occur-prev): New commands.
(occur): Fixed bug preventing line number being displayed if line
number is less than the number of lines of context.