+/*
+ * function: change_field
+ *
+ * parameters:
+ * (char *msg)
+ * message to display as a prompt
+ * (char **field)
+ * a pointer to a pointer which will point a new string. if the latter
+ * pointer != NULL it will be freed (if user doesn't cancel)
+ *
+ * returns (int)
+ * a nonzero value if user has cancelled and zero if user has typed a
+ * valid string
+ */
+