[#2028] Force valid windows drive letters.


Problem description
===================

While working with symlinks on windows I have discovered a strange behavior in Windows API.

in a path in format `c:\some` 'c' is recognized as the drive letter, but for
`complicated-drive:\some` it is recognize as a relative path so you can get `c:\bla\complicated-drive:\some` ... this is inconsistent behavior for me and we need to avoid it as we can hit ugly bugs in the future.


Why we got into this (5 whys)
=============================

There were not tests for it and only recently we added symlink support.

Changes description
===================

Add an explicit test where problem was initially observed.

Raise OSError for getRealPathFromSegments when path has a bad drive.


How to try and test the changes
===============================

reviewers @alibotean 
dependes-on: #26 

Check that code is ok and tests are complete.

See last commit.

Thanks!