Move server API concept to base framework

This commit introduces the new `Component` interface in the form of the
headers base/component.h and base/entrypoint.h. The os/server.h API
has become merely a compatibilty wrapper and will eventually be removed.
The same holds true for os/signal_rpc_dispatcher.h. The mechanism has
moved to base/signal.h and is now called 'Signal_handler'.

Since the patch shuffles headers around, please do a 'make clean' in the
build directory.

Issue #1832

repos/base/src/lib/startup/init_main_thread.cc

@@ -25,7 +25,7 @@ addr_t init_main_thread_result;
 
 extern void init_exception_handling();
 
-namespace Genode { Rm_session * env_stack_area_rm_session(); }
+namespace Genode { extern Rm_session * const env_stack_area_rm_session; }
 
 void prepare_init_main_thread();
 
@@ -93,12 +93,6 @@ extern "C" void init_main_thread()
 	/* initialize exception handling */
 	init_exception_handling();
 
-	/*
-	 * We create the thread-context area as early as possible to prevent other
-	 * mappings from occupying the predefined virtual-memory region.
-	 */
-	env_stack_area_rm_session();
-
 	/*
 	 * Trigger first exception. This step has two purposes.
 	 * First, it enables us to detect problems related to exception handling as