safeguard the synchronized allocator template

* Move the Synced_interface from os -> base
* Align the naming of "synchronized" helpers to "Synced_*"
* Move Synced_range_allocator to core's private headers
* Remove the raw() and lock() members from Synced_allocator and
  Synced_range_allocator, and re-use the Synced_interface for them
* Make core's Mapped_mem_allocator a friend class of Synced_range_allocator
  to enable the needed "unsafe" access of its physical and virtual allocators

Fix #1697

repos/base-linux/src/core/include/platform.h

@@ -15,13 +15,13 @@
 #ifndef _CORE__INCLUDE__LINUX__PLATFORM_H_
 #define _CORE__INCLUDE__LINUX__PLATFORM_H_
 
-#include <base/sync_allocator.h>
 #include <base/allocator_avl.h>
 #include <base/lock_guard.h>
 
 #include <platform_generic.h>
 #include <platform_pd.h>
 #include <platform_thread.h>
+#include <synced_range_allocator.h>
 
 namespace Genode {
 
@@ -34,7 +34,7 @@ namespace Genode {
 			/**
 			 * Allocator for core-internal meta data
 			 */
-			Synchronized_range_allocator<Allocator_avl> _core_mem_alloc;
+			Synced_range_allocator<Allocator_avl> _core_mem_alloc;
 
 			/**
 			 * Allocator for pseudo physical memory

repos/base-linux/src/core/platform.cc

@@ -84,7 +84,7 @@ static void sigchld_handler(int signnum)
 
 
 Platform::Platform()
-: _core_mem_alloc(0)
+: _core_mem_alloc(nullptr)
 {
 	/* catch control-c */
 	lx_sigaction(LX_SIGINT, sigint_handler);