Backporting character_set_filesystem from 5.0 to 5.1.


sql/mysqld.cc:
  Backporting character_set_filesystem from 5.0 to 5.1.
sql/set_var.cc:
  Backporting character_set_filesystem from 5.0 to 5.1.
sql/set_var.h:
  Backporting character_set_filesystem from 5.0 to 5.1.
sql/sql_class.cc:
  Backporting character_set_filesystem from 5.0 to 5.1.
sql/sql_class.h:
  Backporting character_set_filesystem from 5.0 to 5.1.
sql/sql_yacc.yy:
  Backporting character_set_filesystem from 5.0 to 5.1.
mysql-test/t/variables.test:
  Backporting character_set_filesystem from 5.0 to 5.1.
mysql-test/r/variables.result:
  Backporting character_set_filesystem from 5.0 to 5.1.