+20180428
+ + document new form-extension O_EDGE_INSERT_STAY (report by Leon
+ Winter).
+ + correct error-returns listed in manual pages for a few form functions
+ (report by Leon Winter).
+ + add a check in form-library for null-pointer dereference:
+ unfocus_current_field (form);
+ form_driver (form, REQ_VALIDATION);
+ (patch by Leon Winter).
+