Added charset number to result header

Docs/internals.texi:
  Updated protocol information
sql/mysql_priv.h:
  Removed clear_error_message()
sql/mysqld.cc:
  Removed not used function clear_error_message
sql/protocol.cc:
  Simple code cleanup
sql/sql_parse.cc:
  Changed clear_error_message() to thd->clear_error()
  Simple optimization.

include/mysql.h

@@ -84,6 +84,7 @@ typedef struct st_mysql_field {
   unsigned long max_length;   /* Max width of selected set */
   unsigned int flags;         /* Div flags */
   unsigned int decimals;      /* Number of decimals in field */
+  unsigned int charsetnr;     /* Character set */
   enum enum_field_types type; /* Type of field. Se mysql_com.h for types */
 } MYSQL_FIELD;