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-nova/src/base/server/server.cc

@@ -70,6 +70,8 @@ void Rpc_entrypoint::_dissolve(Rpc_object_base *obj)
 	/* make sure nobody is able to find this object */
 	remove(obj);
 
+	/* effectively invalidate the capability used before */
+	obj->cap(Untyped_capability());
 
 	/*
 	 * The activation may execute a blocking operation in a dispatch function.

repos/base-nova/src/core/include/signal_source_component.h

@@ -47,9 +47,6 @@ class Genode::Signal_source_component : public Rpc_object<Nova_signal_source,
 
 		void register_semaphore(Native_capability const &cap)
 		{
-			if (_blocking_semaphore.valid())
-				PWRN("overwritting blocking signal semaphore !!!");
-
 			_blocking_semaphore = cap;
 		}