Kernel: Ignore dirfd if absolute path is given in VFS-related syscalls

To be able to do this, we add a new class called CustodyBase, which can
be resolved on-demand internally in the VirtualFileSystem resolving path
code.

When being resolved, CustodyBase will return a known custody if it was
constructed with such, if that's not the case it will provide the root
custody if the original path is absolute.
Lastly, if that's not the case as well, it will resolve the given dirfd
to provide a Custody object.

Kernel/Tasks/Process.cpp

@@ -1195,6 +1195,11 @@ RefPtr<Custody const> Process::executable() const
     return m_executable.with([](auto& executable) { return executable; });
 }
 
+ErrorOr<NonnullRefPtr<Custody>> Process::custody_for_dirfd(Badge<CustodyBase>, int dirfd)
+{
+    return custody_for_dirfd(dirfd);
+}
+
 ErrorOr<NonnullRefPtr<Custody>> Process::custody_for_dirfd(int dirfd)
 {
     if (dirfd == AT_FDCWD)