Bug#27430 "Crash in subquery code when in PS and table DDL changed after

PREPARE", review fixes:
- make the patch follow the specification of WL#4166 and remove  
the new error that was originally introduced.
Now the client never gets an error from reprepare, unless it failed.
I.e. even if the statement at hand returns a completely different
result set, this is not considered a server error.
The C API library, that can not handle this situation, was modified to
return a client error.
Added additional test coverage.

include/mysql_com.h

@@ -184,19 +184,38 @@ enum enum_server_command
 #define SERVER_MORE_RESULTS_EXISTS 8    /* Multi query - next query exists */
 #define SERVER_QUERY_NO_GOOD_INDEX_USED 16
 #define SERVER_QUERY_NO_INDEX_USED      32
-/*
+/**
   The server was able to fulfill the clients request and opened a
   read-only non-scrollable cursor for a query. This flag comes
   in reply to COM_STMT_EXECUTE and COM_STMT_FETCH commands.
 */
 #define SERVER_STATUS_CURSOR_EXISTS 64
-/*
+/**
   This flag is sent when a read-only cursor is exhausted, in reply to
   COM_STMT_FETCH command.
 */
 #define SERVER_STATUS_LAST_ROW_SENT 128
 #define SERVER_STATUS_DB_DROPPED        256 /* A database was dropped */
 #define SERVER_STATUS_NO_BACKSLASH_ESCAPES 512
+/**
+  Sent to the client if after a prepared statement reprepare
+  we discovered that the new statement returns a different 
+  number of result set columns.
+*/
+#define SERVER_STATUS_METADATA_CHANGED 1024
+
+/**
+  Server status flags that must be cleared when starting
+  execution of a new SQL statement.
+  Flags from this set are only added to the
+  current server status by the execution engine, but 
+  never removed -- the execution engine expects them 
+  to disappear automagically by the next command.
+*/
+#define SERVER_STATUS_CLEAR_SET (SERVER_QUERY_NO_GOOD_INDEX_USED| \
+                                 SERVER_QUERY_NO_INDEX_USED|\
+                                 SERVER_MORE_RESULTS_EXISTS|\
+                                 SERVER_STATUS_METADATA_CHANGED)
 
 #define MYSQL_ERRMSG_SIZE	512
 #define NET_READ_TIMEOUT	30		/* Timeout on read */
@@ -205,6 +224,7 @@ enum enum_server_command
 
 #define ONLY_KILL_QUERY         1
 
+
 struct st_vio;					/* Only C */
 typedef struct st_vio Vio;