clean up and standardize postinstall[32|64].sh scripts

Changelog:

All postinstall32.sh's and postinstall64.sh's are now the same
Autosense if 32-bit Windows, or 64-bit OS
Removed hardcoded references to drive C:
Replaced $VAGRANT_HOME with simply $HOME
Configure ssh using standard Cygwin `ssh-user-config` command
Don't install VMWare Tools, if it's already installed (required for Windows 8.1 32-bit).
Exit with error code of zero (required by Packer)
Do not download files, if they are provided via the template. For example:

"provisioners": [
{
    "type": "file",
    "source": "vagrant.pub",
    "destination": "vagrant.pub"
},
{
    "type": "file",
    "source": "7z922.msi",
    "destination": "7z922.msi"
},

Tested and working for the following boxes:

win2008r2-datacenter
win2008r2-enterprise
win2008r2-standard
win2008r2-web
win2012-standard
win2012r2-standard
win7x64-enterprise
win7x64-pro
win7x86-enterprise
win7x86-pro
win81x64-enterprise
win81x64-pro
win81x86-enterprise
win81x86-pro
win8x64-enterprise
win8x86-enterprise