Use an environment variable to set the location of the configuration directory

Updated code in _include.php to optionally set an alternate configuration directory locaion.  Updated the Memcache Sync, Log clearner and analyzer to accept two optional command line arguments for setting an alternate configuration direcotry location.  The code now will look for the environment variable SIMPLESAMLPHP_CONFIG_DIR for an alternate configuration location.
I don't think $argv can ever contain an empty string.