kadmind: Online LIST using kadm5_iter_principals()

Implement a variation on the op for listing principals where if the
client indicates support for the new variation then we stream the list
instead of collecting it into one reply.  This is the server-side
version of the associated, preceding commit:

    kadm5: Add online kadm5_iter_principals()

kadmin/kadmind.c

@@ -45,6 +45,7 @@ static int debug_flag;
 static int readonly_flag;
 static char *port_str;
 char *realm;
+int list_chunk_size = -1;
 
 static int detach_from_console = -1;
 int daemon_child = -1;
@@ -72,6 +73,9 @@ static struct getargs args[] = {
     {	"debug",	'd',	arg_flag,   &debug_flag,
 	"enable debugging", NULL
     },
+    {   "list-chunk-size", 0,   arg_integer,&list_chunk_size,
+        "set the LIST streaming count of names per chunk", "NUMBER"
+    },
     {
         "detach",       0 ,      arg_flag, &detach_from_console,
         "detach from console", NULL