Fix expand-file-name for names starting with '~'

Fix expand-file-name for names starting with '~'

* src/fileio.c: (file_name_absolute_no_tilde_p):
New static function.
(Fexpand_file_name): If the current buffer's default-directory
starts with "~user" where "user" is not a valid user name, don't
give the '~' a special meaning.  Just treat the value of
default-directory as a relative name.  (Bug#36502)
* test/src/fileio-tests.el
(fileio-tests--relative-default-directory): Add a test.