Added expire-logs-days option 


mysql-test/r/rpl_rotate_logs.result:
  Added test for 'PURGE LOGS BEFORE'
mysql-test/t/rpl_rotate_logs.test:
  Added test for 'PURGE LOGS BEFORE'
sql/log.cc:
  For 'Purge logs before' command
sql/mysqld.cc:
  Added 'expire-logs-days' option
sql/set_var.cc:
  Added 'expire-logs-days' option
sql/sql_class.h:
  For 'Purge logs before' command
sql/sql_parse.cc:
  Added 'expire-logs-days' option
sql/sql_yacc.yy:
  For 'Purge logs before' command