Drop drive letter from path to razor root when RAZOR_ROOT set.

Drop drive letter from path to razor root when RAZOR_ROOT set.

If the RAZOR_ROOT environment variable was set to eg., /root then on
Microsoft Windows we were trying to use paths such as /rootC:/Programs
which is obviously wrong. Instead we should drop the drive letter
giving paths of the form /root/Programs. Note that the drive letter is
_not_ migrated to C:/root/Programs: If a root of C:/root was desired
then RAZOR_ROOT would have been set to C:/root.