Fix a fatal error at startup on Windows due to non-ASCII characters in PATH.

Fix a fatal error at startup on Windows due to non-ASCII characters in PATH.

 main.c (find_and_set_default_shell): Don't use file_exists_p or
 dir_file_exists_p, as those call readdir, which can fail if PATH
 includes directories with non-ASCII characters, and that would
 cause Make to fail at startup with confusing diagnostics.  See
 https://sourceforge.net/mailarchive/message.php?msg_id=30846737
 for the details.