Allow passing a pointer to GetNamedDSMSegment()'s init callback.

 Allow passing a pointer to GetNamedDSMSegment()'s init callback.
 
 This commit adds a new "void *arg" parameter to
 GetNamedDSMSegment() that is passed to the initialization callback
 function.  This is useful for reusing an initialization callback
 function for multiple DSM segments.
 
 Author: Zsolt Parragi <zsolt.parragi@percona.com>
 Reviewed-by: Sami Imseih <samimseih@gmail.com>
 Discussion: https://postgr.es/m/CAN4CZFMjh8TrT9ZhWgjVTzBDkYZi2a84BnZ8bM%2BfLPuq7Cirzg%40mail.gmail.com