main: dump history by default

Always write the measurement history on exit when the dump directory is
specified and silently ignore the dumponexit directive. There doesn't
seem to be a good use case for dumpdir and -r without dumponexit as the
history would be invalidated by adjustments of the clock that happened
between the dump command and chronyd exit.

main.c

@@ -97,7 +97,7 @@ MAI_CleanupAndExit(void)
 {
   if (!initialised) exit(exit_status);
   
-  if (CNF_GetDumpOnExit()) {
+  if (CNF_GetDumpDir()[0] != '\0') {
     SRC_DumpSources();
   }