added cscope-rerun-search-at-point command

This command runs the search at point of the *cscope* buffer again, replacing
the results in-place in the *cscope* buffer. This required some cleanup in the
process filter code to not assume that the results are going to (point-max)
comments
cleaned up some edge cases involving history separator navigation
minor refactoring; no functional changes
nav routines now use regexes instead of properties