Added Unique class to be used for duplicate removal in multi-table delete.

mysys/ptr_cmp.c:
  Removed old comments
pstack/bucomm.c:
  Change to use mkstemp.
sql/Makefile.am:
  Added Unique class
sql/filesort.cc:
  Changes to let the Unique class use the old filesort code.
sql/sql_class.h:
  Added Unique class
sql/sql_handler.cc:
  Removed warning.
sql/sql_select.cc:
  Cleaned up typo.

sql/sql_sort.h

@@ -0,0 +1,54 @@
+/* Copyright (C) 2000 MySQL AB
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2 of the License, 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 General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA */
+
+/* Defines used by filesort and uniques */
+
+#define MERGEBUFF		7
+#define MERGEBUFF2		15
+
+typedef struct st_buffpek {		/* Struktur om sorteringsbuffrarna */
+  my_off_t file_pos;			/* Where we are in the sort file */
+  uchar *base,*key;			/* key pointers */
+  ha_rows count;			/* Number of rows in table */
+  ulong mem_count;			/* numbers of keys in memory */
+  ulong max_keys;			/* Max keys in buffert */
+} BUFFPEK;
+
+
+typedef struct st_sort_param {
+  uint sort_length;			/* Length of sort columns */
+  uint keys;				/* Max keys / buffert */
+  uint ref_length;			/* Length of record ref. */
+  ha_rows max_rows,examined_rows;
+  TABLE *sort_form;			/* For quicker make_sortkey */
+  SORT_FIELD *local_sortorder;
+  SORT_FIELD *end;
+  uchar *unique_buff;
+#ifdef USE_STRCOLL
+  char* tmp_buffer;
+#endif
+} SORTPARAM;
+
+
+int merge_many_buff(SORTPARAM *param, uchar *sort_buffer,
+		    BUFFPEK *buffpek,
+		    uint *maxbuffer, IO_CACHE *t_file);
+uint read_to_buffer(IO_CACHE *fromfile,BUFFPEK *buffpek,
+		    uint sort_length);
+int merge_buffers(SORTPARAM *param,IO_CACHE *from_file,
+		  IO_CACHE *to_file, uchar *sort_buffer,
+		  BUFFPEK *lastbuff,BUFFPEK *Fb,
+		  BUFFPEK *Tb,int flag);