Kernel: Add a Linux-style getrandom syscall

The way it gets the entropy and blasts it to the buffer is pretty
ugly IMHO, but it does work for now. (It should be replaced, by
not truncating a u32.)

It implements an (unused for now) flags argument, like Linux but
instead of OpenBSD's. This is in case we want to distinguish
between entropy sources or any other reason and have to implement
a new syscall later. Of course, learn from Linux's struggles with
entropy sourcing too.

Kernel/Syscall.cpp

@@ -313,6 +313,8 @@ static u32 handle(RegisterDump& regs, u32 function, u32 arg1, u32 arg2, u32 arg3
         return current->process().sys$get_process_name((char*)arg1, (int)arg2);
     case Syscall::SC_realpath:
         return current->process().sys$realpath((const char*)arg1, (char*)arg2, (size_t)arg3);
+    case Syscall::SC_getrandom:
+        return current->process().sys$getrandom((void*)arg1, (size_t)arg2, (unsigned int)arg3);
     default:
         kprintf("<%u> int0x82: Unknown function %u requested {%x, %x, %x}\n", current->process().pid(), function, arg1, arg2, arg3);
         return -ENOSYS;