Cannot access subst drives after commit 456c3a46

[Jeremy Drake]

Sort of like the system32 issue, but still present after the fix for that.

Previously I was able to shorten paths for both native and cygwin
processes by using SUBST to map a drive letter.  After this commit was
applied, this stopped working:

$ subst
T:\: -> C:\blah\blah\MINGW-packages

$ ls /t/
ls: cannot access '/t/': No such file or directory

$ mount
...
T: on /t type ntfs (binary,noacl,posix=0,user,noumount,auto)