client/mysqlslap.c:
  compilation errors
include/plugin.h:
  comments corrected
sql/sql_plugin.cc:
  LEX_STRING instead of char*,
  MYSQL_HANDLERTON_INTERFACE_VERSION instead of 0x0000
sql/sql_plugin.h:
  LEX_STRING instead of char*
sql/sql_show.cc:
  LEX_STRING instead of char*.
  STRING_WITH_LEN works only on string literals, not on expressions