Cygwin support

As discussed in #506, Cygwin requires *native <-> Cygwin* filename translations.

In this new pull request  default translation functions are provided for Cygwin systems, so users don't have to setup this.

I also considered using *defcustom* first. But I think the additional custom variable would confuse users. Because most users never need to custimize this.

I use some elisp code to programmatically/conditionally rebind this vars for some advanced setups (local mounted windows share of my CLJ-code). I dont see any use-case for a **static** customization.


Slime also doesn't use *defcustom* for this case.