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-foc/include/signal_source/client.h

@@ -70,6 +70,13 @@ namespace Genode {
 			: Rpc_client<Foc_signal_source>(static_cap_cast<Foc_signal_source>(cap))
 			{ _init_sem(); }
 
+			/**
+			 * Destructor
+			 */
+			~Signal_source_client()
+			{
+				Fiasco::l4_irq_detach(_sem.dst());
+			}
 
 			/*****************************
 			 ** Signal source interface **