The SHOW VARIABLES LIKE .../SELECT @@/SELECT ... FROM INFORMATION_SCHEMA.VARIABLES
were assuming that all the system variables are in system charset (UTF-8).
However the variables that are settable through command line will have a different
character set (character_set_filesystem).
Fixed the server to remember the correct character set of basedir, datadir, tmpdir,
ssl, plugin_dir, slave_load_tmpdir, innodb variables; init_connect and init_slave 
variables and use it when processing data.

mysql-test/r/ctype_filesystem.result:
  Bug #37339: test case (should be in utf-8)
mysql-test/t/ctype_filesystem-master.opt:
  Bug #37339: test case (should be in ISO-8859-1)
mysql-test/t/ctype_filesystem.test:
  Bug #37339: test case
sql/mysqld.cc:
  Bug #37339: remember the correct character set for init_slave and init_connect
sql/set_var.cc:
  Bug #37339: 
    - remember the character set of the relevant variables
    - implement storing and using the correct 
      character set
sql/set_var.h:
  Bug #37339: implement storing and using the correct 
  character set
sql/sql_show.cc:
  Bug #37339: implement storing and using the correct 
  character set