Bazel client, JNI, Windows: impl. CreateJunction

Implement a CreateJunction function in the Windows
JNI library. Also move a bit of code from
file_windows to the JNI library, where it is
(also) needed.

This implementation is an improved version of
`blaze_util::SymlinkDirectories` in
blaze_util_windows: this version handles Windows
paths as `name` and `target`, and performs more
validation (e.g. on the length of `target`), plus
has more comments explaining the logic. In a
subsequent change I'll start using this new
function in blaze_util_windows.

This method will also be helpful in tests: we will
no longer have to shell out to mklink.

See https://github.com/bazelbuild/bazel/issues/2107

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Change-Id: I7e9b085fdc2ba47be83da5319bded02bd323e71b
Reviewed-on: https://cr.bazel.build/8892
PiperOrigin-RevId: 147585207
MOS_MIGRATED_REVID=147585207

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