New file. Handles "require" statement

+/* GNU mailutils - a suite of utilities for electronic mail
+   Copyright (C) 1999, 2000, 2001, 2002 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU Lesser General Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.  */
+
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif  
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>  
+#include <string.h>  
+#include <sieve.h>
+
+void
+sieve_require (list_t slist)
+{
+  int status;
+  iterator_t itr;
+  
+  status = iterator_create (&itr, slist);
+  if (status)
+    {
+      sieve_error ("%s:%d: cannot create iterator: %s",
+		   sieve_filename, sieve_line_num,
+		   mu_errstring (status));
+      return;
+    }
+
+  for (iterator_first (itr); !iterator_is_done (itr); iterator_next (itr))
+    {
+      char *s;
+      sieve_register_t *reg;
+      
+      iterator_current (itr, (void **)&s);
+
+      /* FIXME: if (strncmp (s, "comparator-", 11) ... */
+      reg = sieve_action_lookup (s);
+      if (!reg)
+	{
+	  sieve_error ("%s:%d: source for the required action %s is not available",
+		       sieve_filename, sieve_line_num, s);
+	  break;
+	}
+      reg->required = 1;
+    }
+  iterator_destroy (&itr);
+}
+