Kernel: Migrate sys$unveil to use the KString API

This avoids potential unhandled OOM that's possible with the old
copy_string_from_user API.

Kernel/Syscalls/unveil.cpp

@@ -53,13 +53,17 @@ KResultOr<FlatPtr> Process::sys$unveil(Userspace<const Syscall::SC_unveil_params
  if (path.is_empty() || !path.view().starts_with('/'))
  return EINVAL;
 
- auto permissions = copy_string_from_user(params.permissions);
- if (permissions.is_null())
- return EFAULT;
+ OwnPtr<KString> permissions;
+ {
+ auto permissions_or_error = try_copy_kstring_from_user(params.permissions);
+ if (permissions_or_error.is_error())
+ return permissions_or_error.error();
+ permissions = permissions_or_error.release_value();
+ }
 
  // Let's work out permissions first...
  unsigned new_permissions = 0;
- for (const char permission : permissions) {
+ for (const char permission : permissions->view()) {
  switch (permission) {
  case 'r':
  new_permissions |= UnveilAccess::Read;