Allow dynamic allocation of shared memory segments.

Patch by myself and Amit Kapila.  Design help from Noah Misch.  Review
by Andres Freund.

src/include/portability/mem.h

@@ -0,0 +1,40 @@
+/*-------------------------------------------------------------------------
+ *
+ * mem.h
+ *	  portability definitions for various memory operations
+ *
+ * Copyright (c) 2001-2013, PostgreSQL Global Development Group
+ *
+ * src/include/portability/mem.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef MEM_H
+#define MEM_H
+
+#define IPCProtection	(0600)	/* access/modify by user only */
+
+#ifdef SHM_SHARE_MMU			/* use intimate shared memory on Solaris */
+#define PG_SHMAT_FLAGS			SHM_SHARE_MMU
+#else
+#define PG_SHMAT_FLAGS			0
+#endif
+
+/* Linux prefers MAP_ANONYMOUS, but the flag is called MAP_ANON on other systems. */
+#ifndef MAP_ANONYMOUS
+#define MAP_ANONYMOUS			MAP_ANON
+#endif
+
+/* BSD-derived systems have MAP_HASSEMAPHORE, but it's not present (or needed) on Linux. */
+#ifndef MAP_HASSEMAPHORE
+#define MAP_HASSEMAPHORE		0
+#endif
+
+#define PG_MMAP_FLAGS			(MAP_SHARED|MAP_ANONYMOUS|MAP_HASSEMAPHORE)
+
+/* Some really old systems don't define MAP_FAILED. */
+#ifndef MAP_FAILED
+#define MAP_FAILED ((void *) -1)
+#endif
+
+#endif   /* MEM_H */