VSI*L and "errno" setting.

[warmerdam]

In order to enable higher level code to report errors from the various
VSI*L functions such as VSIFOpenL() it is necessary that "errno" get set. 
This does not currently occur on win32 (with cpl_vsil_win32.cpp code). 

Minimally the Open() method in cpl_vsil_win32.cpp should try to translate
win32 error codes to corresponding errno values.  Maximally error setting
should be done consistently through cpl_vsil_win32.cpp and cpl_vsi_mem.cpp
along with some effort to test these in the test suite.