Add exec option to search

BRL-CAD's MGED search needs an -exec option - like the find command in UNIX - but that would mean low level library functions accessing the high-level Tcl interpreter in MGED. The solution is to provide the database search routine with a callback and a userdata pointer. My task is to add the callback logic, extend the parser, write the callback for MGED, extend the evaluation of search plans and write tests and documentation for all changes.