core: add Platform::max_caps()

This method returns the kernel-specific system-global limit of the total
number of capabilities.

Issue #2398

repos/base-foc/src/include/base/internal/cap_alloc.h

@@ -134,6 +134,8 @@ namespace Genode {
 			{
 				construct_at<Cap_index_allocator_tpl<T, SZ> >(this);
 			}
+
+			size_t max_caps() const override { return SZ; }
 	};
 }
 

repos/base-foc/src/include/base/internal/cap_map.h

@@ -113,6 +113,11 @@ namespace Genode {
 			 * Redo construction of the object
 			 */
 			virtual void reinit() = 0;
+
+			/**
+			 * Return capacity of allocator
+			 */
+			virtual size_t max_caps() const = 0;
 	};