flaws list)
TODO: 
 * verify that no sequence of API calls produces SIGSEGV.
 That is, verify that mysql_stmt_init  -> mysql_stmt_fetch is OK,
 or mysql_stmt_prepare -> mysql_stmt_fetch_column is OK and sets
 meaningful error.
 * remove alloc_stmt_fields call
 * revise stmt->state codes and statement states.
 * there are other items in prepared statements 'to fix' document.

Done:
 - cleanups and comments
 - revision of prepared statement error codes.
 - mysql_stmt_prepare is now can always be called (that is, you can reprepare
   a statement)
 - new implementation of mysql_stmt_close and fetch cancellation


include/errmsg.h:
  - CR_NOT_ALL_PARAMS_BOUND - this error code wasn't used until now. 
    Apparently it was added in advance, but then interface of
    mysql_stmt_bind_param changed. Now it's not possible to bind only some
    parameters - either all or none of parameters are bound.
    This error code  is renamed to CR_PARAMS_NOT_BOUND
  - CR_FETCH_CANCELLED - error code set on server side when fetch from 
    MYSQL_RES or MYSQL_STMT (in blocking mode) was cancelled because of
    intercepting call to mysql_stmt_close
  - CR_NO_DATA - this is proposed error code to return from 
    mysql_stmt_fetch_column if no row was fetched (by any type of fetch).
    We always can fall back to CR_COMMANDS_OUT_OF_SYNC though.
    Need reviewer's opinion on this one.
include/mysql.h:
  - added unbuffered_fetch_owner member to MYSQL to point to MYSQL_RES 
    or MYSQL_STMT which is used to fetch result at the moment.
    This is to be able to set CR_FETCH_CANCELLED error without fantoms.
  - added unbuffered_fetch_cancelled boolean variable to MYSQL_STMT and 
    MYSQL_RES structures
  - rename PREP_STMT_STATE -> enum enum_mysql_stmt_state
  - members of MYSQL_STMT ordered by size.
  - removed members of MYSQL_STMT: current_row, result_buffered,
    last_fetched_column, last_fetched_buffer, query
  - renamed members of MYSQL_STMT: param_buffers -> bind_param_done, 
    res_buffers -> bind_result_done
  - now mysql_stmt_fetch calls stmt->read_row_func to read row either from 
    buffer or from network.
include/sql_common.h:
  declaration for flush_use_result
libmysql/client_settings.h:
  stmt_close declaration removed
libmysql/errmsg.c:
  Error messages for changed and added error codes.
libmysql/libmysql.c:
  Many changes:
  - some unused variables removed
  - cleanups
  - better error reporting
  - some function calls commented
  - alloc_stmt_fields is now called right after execute, to not read
    mysql->fields of some other statement
  - new implementation of mysql_stmt_fetch - this is also with cursor
    fetch in mind (to implement cursor fetch I'll just need to write
    special read_row function for it, so this change will be local)
  - implementation of fetch cancellation, including complete rewrite of
    mysql_stmt_close
  - now mysql_stmt_free_result doesn't free results of other statements.
sql-common/client.c:
  - implementation of flush_use_result
  - implementation of fetch cancellation
  - changed behaviour of mysql_close in regard to mysql_stmt_close - now 
    mysql_close just set stmt->mysql to 0