Use UNC names for network paths in WOK on Windows

Currently WOK configuration on Windows refers to network disks using disk
letters like H: for //shamox/dn01 (see, for instance,
//aquarex/sbu/KAS/dev/adm/ros.edl).

This assumes that appropriate disk mapping must be set on workstation when WOK
is used. It is a problem bacause:

- it is far from being obvious which disk corresponds to each name, so we always
have to ask somebody. Note that many disks contain similar subdirectories (like
KAS/dev/ros), so it is pretty easy to make mistake in disk mapping, which could
be hard to recognize.

- in some cases that disk letter can be already used by another disk.

To fix this annoying problem, it should be possible to use UNC names for network
paths for workbenches, for instance, //shamox/dno1 instead of H:.

This seems to be possible, at least compilation and 'wokenv -s' commands work
well (tested on workbench KAS:dev:abvmt, see file
//aquarex/sbu/KAS/dev/adm/abvmt.edl).
The problem currently exists with Microsoft's linker (LINK), which recognizes
paths starting from shash as option. To avoid this, we can convert / to \ in
linker command line.