Allow dynamic allocation of shared memory segments.

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

src/backend/utils/misc/postgresql.conf.sample

@@ -123,6 +123,13 @@
 #work_mem = 1MB				# min 64kB
 #maintenance_work_mem = 16MB		# min 1MB
 #max_stack_depth = 2MB			# min 100kB
+#dynamic_shared_memory_type = posix # the default is the first option
+					# supported by the operating system:
+					#   posix
+					#   sysv
+					#   windows
+					#   mmap
+					# use none to disable dynamic shared memory
 
 # - Disk -