Bug#15126 character_set_database is not replicated (LOAD DATA INFILE need it)

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.

sql/sql_class.h

@@ -1687,6 +1687,7 @@ public:
   bool opt_enclosed;
   bool dumpfile;
   ulong skip_lines;
+  CHARSET_INFO *cs;
   sql_exchange(char *name,bool dumpfile_flag);
 };