[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()

modules.yaml

@@ -43,7 +43,6 @@ modules:
             - src/kernel/objects/thread.cpp
             - src/kernel/objects/process.cpp
             - src/kernel/objects/vm_area.cpp
-            - src/kernel/page_manager.cpp
             - src/kernel/page_table.cpp
             - src/kernel/pci.cpp
             - src/kernel/scheduler.cpp
@@ -59,6 +58,7 @@ modules:
             - src/kernel/syscalls/process.cpp
             - src/kernel/syscalls/system.cpp
             - src/kernel/syscalls/thread.cpp
+            - src/kernel/syscalls/vm_area.cpp
             - src/kernel/task.s
             - src/kernel/vm_space.cpp
             - src/kernel/crtn.s