Add pg_shmem_allocations view.

This tells you about allocations that have been made from the main
shared memory segment. The original patch also tried to show information
about dynamic shared memory allocation as well, but I decided to
leave that problem for another time.

Andres Freund and Robert Haas, reviewed by Michael Paquier, Marti
Raudsepp, Tom Lane, Álvaro Herrera, and Kyotaro Horiguchi.

Discussion: http://postgr.es/m/20140504114417.GM12715@awork2.anarazel.de

src/include/storage/shmem.h

@@ -59,7 +59,8 @@ typedef struct
 {
 	char		key[SHMEM_INDEX_KEYSIZE];	/* string name */
 	void	   *location;		/* location in shared mem */
-	Size		size;			/* # bytes allocated for the structure */
+	Size		size;			/* # bytes requested for the structure */
+	Size		allocated_size;	/* # bytes actually allocated */
 } ShmemIndexEnt;
 
 /*