ncurses 6.1 - patch 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).