Misc bugfixes

commit 1726a115:

cerbero: Fix occasional PermissionError on Windows

During extract, sometimes we get a PermissionError when renaming a newly-extracted source tree because the Anti-Virus is scanning it. Try to account for that.

commit 6bd9a9ba:

cerbero: Force python scripts to output continuously

When we redirect output to a file, Python uses fully-buffered output and does not write anything till EOF. This is bad for following the configure log in real-time.

Fixes #122 (closed)

commit b2a05b21:

cerbero: Auto-detect when the console is interactive

The default setting for this should not be True. The default setting should auto-detect it. Setting it through the config continues to work as before.

commit 5ca6b3cc:

cerbero: Use variant for export MSVC shell

We were using a separate winXX-msvc-shell.cbc configuration file to signal to the main configuration file that we want to export for an MSVC shell on Windows.

This meant that people had to first know that they were supposed to use this, and then know that they could chain it with other configuration files to get a shell with, f.ex., a different prefix.

We already have a mechanism for picking msvc vs mingw shell: the variant. So now we use that.