Kernel: Have the open() syscall take an explicit path length parameter.

Instead of computing the path length inside the syscall handler, let the
caller do that work. This allows us to implement to new variants of open()
and creat(), called open_with_path_length() and creat_with_path_length().
These are suitable for use with e.g StringView.

Kernel/Syscall.h

@@ -148,6 +148,13 @@ struct SC_mmap_params {
     const char* name { nullptr };
 };
 
+struct SC_open_params {
+    const char* path;
+    int path_length;
+    int options;
+    u16 mode;
+};
+
 struct SC_select_params {
     int nfds;
     fd_set* readfds;