to make list of parameters in mysql_alter_table shorted
to avoid warning in MSVC (windows) building 


sql/mysql_priv.h:
  Introduced parameter object "ALTER_INFO" for mysql_alter_table
  - changed declaration of mysql_alter_table
  - changed declaration of mysql_drop_index 
    (replaced List<Alter_drop> by ALTER_INFO)
sql/sql_lex.h:
  Introduced parameter object "ALTER_INFO" for mysql_alter_table
  - declared struct ALTER_INFO
  - added ALTER_INFO alter_info; into LEX
  - removed fields which were added into ALTER_INFO from LEX
sql/sql_parse.cc:
  Introduced parameter object "ALTER_INFO" for mysql_alter_table
  - changed invotaions of mysql_alter_table
  - replaced cleaning fields in SQLCOM_OPTIMIZE by ALTER_INFO::reset
  - changed invocation of mysql_drop_index (replaced lex->drop_list by &lex->alter_info)
  - changed definition of mysql_create_index and mysql_drop_index to use ALTER_INFO
sql/sql_table.cc:
  Introduced parameter object "ALTER_INFO" for mysql_alter_table
  - changed definition of mysql_alter_table
sql/sql_yacc.yy:
  Introduced parameter object "ALTER_INFO" for mysql_alter_table
  - replaced using of removed from LEX fields my fields of LEX::alter_info