FileTree shouldn't generate reserved names on Windows

[peteruhnak]

Windows has a list of reserved file names and patterns https://msdn.microsoft.com/en-us/library/aa365247(VS.85).aspx#naming_conventions

more specifically this:

CON, PRN, AUX, NUL, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9, LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, and LPT9

I ran into this issue, because I had methods aux and aux:, which would on linux generate aux.st and ux..st, but on windows no such files can be created.