[kernel] Add test mode, controlled by manifest

The manifest can now supply a list of boot flags, including "test".
Those get turned into the bootproto::args::flags field by the
bootloader. The kernel takes those and uses the test flag to control
enabling syscalls with the new "test" attribute, like the new
test_finish syscall, which lets automated tests call back to the kernel
to shut down the system.

definitions/syscalls.def

@@ -42,4 +42,9 @@ interface syscalls [syscall] {
         param clone ref object [out]             # The new handle
         param mask uint32                        # The capability bitmask
     }
+
+    # Testing mode only: Have the kernel finish and exit QEMU with the given exit code
+    function test_finish [test] {
+        param exit_code uint32
+    }
 }