makefs: Fix cd9660 filename buffer maximum length

[emaste]

Summary

The maximum length is 30 characters for name and extension, two
separators (. and ;) and 5 characters for file version from 1 to 32767,
which is 37 characters.  Add one for the null term as we treat these
buffers as C strings.

This is not an issue in practice, as the file version is always 1 in
makefs.

While here, drop `_WITH_PADDING` from the macro name and update the
previously-unused ISO_FILENAME_MAXLENGTH for the corrected length.
A 0x00 padding byte is used by ISO9660 when needed for alignment, which
can be the null byte at the end of the string.

[kevans]

Most or all of these uses would seem to be better as sizeof(foo) instead of the underlying constant IMO

[emaste]

Use sizeof

[kevans]

Thanks!