Fix decoding of directories when "~" includes non-ASCII chars

Fix decoding of directories when "~" includes non-ASCII chars

* src/fileio.c (Fexpand_file_name): Don't build multibyte strings
from unibyte non-ASCII strings when NAME and DEFAULT_DIRECTORY
have different multibyteness, as this adds bytes to the byte
sequence, and in some situations, e.g., when the home directory
includes non-ASCII characters, can fail file APIs.  (Bug#30755)

* lisp/startup.el (normal-top-level): Make sure default-directory
is set to a multibyte string when decoded on MS-Windows.

(cherry picked from commit 3aab8626ba5080bb04d0fdae52d99c850a842a52)