(FILESYSTEM_PREFIX_LEN): Define.

(FILESYSTEM_PREFIX_LEN): Define.
(dir_name_r): Declare this function as static.
[BACKSLASH_IS_PATH_SEPARATOR]: Fix a bug that'd
manifest itself on a name containing a mix of slashes and
backslashes.
Make this function work with names starting with a DOS-style
drive letter and colon prefix.
(dir_name): Append `.' if necessary.
Based mostly on patches from Prashant TR and Eli Zaretskii.