Fixed memory leaks and compiler warnings in ha_sphinx.cc
Added HA_MUST_USE_TABLE_CONDITION_PUSHDOWN so that an engine can force index condition to be used

mysql-test/suite/sphinx/sphinx.result:
  Added testing of pushdown conditions and sphinx status variables.
mysql-test/suite/sphinx/sphinx.test:
  Added testing of pushdown conditions and sphinx status variables.
mysql-test/suite/sphinx/suite.pm:
  Print version number if sphinx version is too old.
sql/handler.h:
  Added HA_MUST_USE_TABLE_CONDITION_PUSHDOWN so that an engine can force index condition to be used
sql/sql_base.cc:
  Added 'thd' argument to check_unused() to be able to set 'entry->in_use' if we call handler->extra().
  This was needed as sphinx (and possible other storage engines) assumes that 'in_use' is set if handler functions are called.
sql/sql_select.cc:
  Test if handler is forcing pushdown condition to be used.
storage/sphinx/ha_sphinx.cc:
  Updated to version 2.0.4
  Fixed memory leaks and compiler warnings.
storage/sphinx/ha_sphinx.h:
  Updated to version 2.0.4
storage/sphinx/snippets_udf.cc:
  Updated to version 2.0.4