filename -- support case insensitivity in filename

[Fred T. Hamster]

support case insensitivity:

1. the case sensitivity issue should be taken to the dirname and filename classes; they should be able to enable / disable the comparisons' case sensitivity.
2. that's the realm of OS where you need to consider this, but in general, two strings of different case ARE different; in mathematical terms, like the comparisons of numbers.
3. but in certain OS area, it makes sense to disable case sensitivity.