[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.

src/kernel/syscall.h.cog

@@ -17,5 +17,5 @@ cog.outl(f"constexpr size_t num_syscalls = {len(syscalls.methods)};")
 ]]]*/
 /// [[[end]]]
 
-void syscall_initialize();
+void syscall_initialize(bool enable_test);
 extern "C" void syscall_enable();