ENHANCED: use terms at point as "future history" for term-search

ENHANCED: use terms at point as "future history" for term-search

* sweep.pl (sweep_terms_at_point/2): new predicate.
* sweeprolog.el (sweeprolog-terms-at-point): new function.
(sweeprolog-read-term): use it for setting the future history.
(sweeprolog-read-goal): new function.
(sweeprolog-term-search): use it for reading a goal when called with
prefix argument.