Using a response file will eliminate the headache associated with batch argument/quote processing.
For example I unsucessfully compiled python with visualcpptools when specifying VSInstallDir in the batch file (cannot find vcruntime.h)
```batch
build.bat -p x64 -e -M --no-tkinter "/p:VCInstallDir=%VCInstallDir%"
```
but it build successfully when specifying it in a response file
msbuild.rsp:
```
/p:VCInstallDir=%VCInstallDir%
```
This modifies the behavior of the '-e' flag to PCbuild\build.bat: when '-e'
is not supplied, no attempt will be made to build extension modules that
require external libraries, even if the external libraries are present.
Also adds '--no-<module>' flags to PCbuild\build.bat, where '<module>' is
one of 'ssl', 'tkinter', or 'bsddb', to allow skipping just those modules
(if '-e' is given).
Rather than using Burn "Persisted" variables we now add registry keys for each added feature. These can be detected by the installer regardless of which version installed them, and we use this for Modify and Upgrade. In particular, Upgrades can't access the Persisted variables, but can find well-known registry keys.
There are also some changes to the bootstrap app to properly handle upgrades.
Finally, a few minor improvements to the Windows build to keep things tidier.
