This patch fixes problem that LOAD DATA could use different
character sets when loading files on master and on slave sides:
- Adding replication of thd->variables.collation_database
- Adding optional character set clause into LOAD DATA

Note, the second way, with explicit CHARACTER SET clause
should be the recommended way to load data using an alternative
character set.
The old way, using "SET @@character_set_database=xxx" should be
gradually depricated.


mysql-test/r/mysqlbinlog.result:
  Adding test case
mysql-test/t/mysqlbinlog.test:
  Adding test case
sql/log_event.cc:
  Adding logging of thd->variables.collation_database
sql/log_event.h:
  Adding declarations
sql/sql_class.cc:
  Exchange character set is null by default
sql/sql_class.h:
  Adding character set into sql_exchange
sql/sql_load.cc:
  - Using exchange character set (if it was specified in LOAD DATA syntax)
  - Using thd->variables.collation_database by default
sql/sql_yacc.yy:
  Adding optional character set clause into LOAD DATA syntax
mysql-test/r/rpl_loaddata2.result:
  New BitKeeper file ``mysql-test/r/rpl_loaddata2.result''
mysql-test/std_data/loaddata6.dat:
  New BitKeeper file ``mysql-test/std_data/loaddata6.dat''
mysql-test/t/rpl_loaddata2.test:
  New BitKeeper file ``mysql-test/t/rpl_loaddata2.test''