client/mysql.cc:
  A fix (bug #4802 prompt in mysql client shows wrong database after dropping default db).
  Introduced new get_current_db() function which is called from the com_use()
  and the com_go() if we get SERVER_STATUS_DB_DROPPED.
include/mysql_com.h:
  A fix (bug #4802 prompt in mysql client shows wrong database after dropping default db).
  SERVER_STATUS_DB_DROPPED flag added.
    Note: it is set to 256 to don't conflict with 5.0 ver.
sql/sql_db.cc:
  A fix (bug #4802 prompt in mysql client shows wrong database after dropping default db).
  SERVER_STATUS_DB_DROPPED flag is set/unset.