Just like we support having alternates pointing to different drives, we
want to support alternates pointing to network shares, i.e. UNC paths.
Technically, what we do in this patch is not to support UNC alternates,
but to support UNC paths when normalizing paths. But the latter implies
the former, and the former really was the motivation for this patch.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>