unitsync changes for next release

[Tobi]

* Add GetVersion() function to unitsync. This returns the Spring version that
included this unitsync DLL/SO for the first time (ie. not necessarily the
current Spring version, that depends on whether unitsync has been recompiled.)

* Remove InitArchiveScanner() unitsync function. It was a no-op since 0.74b1
or earlier; now there should be sufficient time for everyone to stop using it.

[Tobi]

* Windows apparently pollutes the global namespace with a GetVersion() function already,
so I renamed it to GetSpringVersion(). Semantics are the same.