pginstaller - pginst: Make sure CAs are only called when on NT

Log Message:
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Make sure CAs are only called when on NT platforms, since the DLL won't load on 9x. (They're only used for server
installs)

Get rid of crufty registry check for windows version, and use the installer prodvided property instead.

Modified Files:
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    pginst/wxs:
        dialogs.wxs (r1.29 -> r1.30)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pginstaller/pginst/wxs/dialogs.wxs.diff?r1=1.29&r2=1.30)
        pginst.wxs (r1.69 -> r1.70)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pginstaller/pginst/wxs/pginst.wxs.diff?r1=1.69&r2=1.70)