Fix semantics of "general."{systemdialogs,progressbar}

These were renamed from "skip_"* to *. However, we only changed syntax
and didn't invert their semantic meaning. Fix this by negating the
semantics of those variables. Also, negate these in the configuration
file automatically when running `calcurse-upgrade`.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/help.c

@@ -380,8 +380,8 @@ help_screen (void)
     _("Import data from an icalendar file.\n"
       "You will be asked to enter the file name from which to load ical\n"
       "items. At the end of the import process, and if the general option\n"
-      "'skip_system_dialogs' is not set to 'yes', a report indicating how\n"
-      "many items were imported is shown.\n"
+      "'system_dialogs' is set to 'yes', a report indicating how many items\n"
+      "were imported is shown.\n"
       "This report contains the total number of lines read, the number of\n"
       "appointments, events and todo items which were successfully imported,\n"
       "together with the number of items for which problems occured and that\n"