conf: rework local directive to have default stratum

Allow the local directive to be specified without the stratum field.
It's an option now, with default value 10. Also, move the parsing code
to cmdparse.c to make it available to the client.

conf.c

@@ -817,13 +817,9 @@ parse_log(char *line)
 static void
 parse_local(char *line)
 {
-  int stratum;
-  if (sscanf(line, "stratum%d", &stratum) == 1) {
-    local_stratum = stratum;
-    enable_local = 1;
-  } else {
+  if (!CPS_ParseLocal(line, &local_stratum))
     command_parse_error();
-  }
+  enable_local = 1;
 }
 
 /* ================================================== */