[kernel] Add VMA interface

Finished the VMA kobject and added the related syscalls. Processes can
now allocate memory! Other changes in this commit:

- stop using g_frame_allocator and add frame_allocator::get()
- make sure to release all handles in the process dtor
- fix kutil::map::iterator never comparing to end()

src/kernel/vm_space.h

@@ -48,6 +48,13 @@ public:
 	static vm_space & kernel_space();
 
 	/// Copy a range of mappings from the given address space
+	/// \arg source The address space that already contains the mappings
+	/// \arg from   The starting virtual address in the source address space
+	/// \arg to     The starting virtual address in this address space
+	/// \arg count  The number of page entries to copy
+	void copy_from(const vm_space &source, uintptr_t from, uintptr_t to, size_t count);
+
+	/// Map virtual addressses to the given physical pages
 	/// \arg virt  The starting virutal address
 	/// \arg phys  The starting physical address
 	/// \arg count The number of contiugous physical pages to map