Added:

PURGE [MASTER] LOGS BEFORE date/date_expression
expire-logs-days option
  With this option old files are deleted when
      - mysqld is started
      - log is rotated
      - someone does FLUSH LOGS






sql/lex.h:
  For PURGE LOGS BEFORE
sql/log.cc:
  Added purge_logs_before_date to MYSQL_LOG class
sql/mysql_priv.h:
  For startup option --expire-logs-days
sql/mysqld.cc:
  Added options expire-logs-days
  When mysqld is started old log files are deleted.
sql/sql_class.h:
  For PURGE LOGS BEFORE and expire-log-days option
sql/sql_lex.h:
  For PURGE LOGS BEFORE
sql/sql_parse.cc:
  Old logs are deleted
  when someone does FLUSH LOGS
sql/sql_repl.cc:
  For PURGE LOGS BEFORE and expire-log-days option
sql/sql_repl.h:
  For PURGE LOGS BEFORE and expire-log-days option
sql/sql_yacc.yy:
  For PURGE LOGS BEFORE