Use mklink under Windows

In order to support in-place importing on Windows, mklink
rather than ln must be used. Rather than create new commands
like `--transfer=mklink`, the same flags (`ln` and `ln_s`)
are being used.

Note: in working through this PR, it was discovered that the
"active closing" of `RawFileStore` proxies which is required
to have in-place import work on Windows, broke compatibility
with earlier 5.0.x server versions. As a compromise, active
closing is now off by default and must be turned on from the
command-line. In 5.1, the default will be to enable active
closing.

/cc @bpindelski