This replaces built-in os.MkdirAll with a patched version that stops the recursion
when reaching the volume part of the path. The original version would continue recursion,
and for extended length paths end up with \\? as the top-level directory, and the error
message would then be something like:
mkdir \\?: The filename, directory name, or volume label syntax is incorrect.
This was factored from fstest as we were including the testing
enviroment into the main binary because of it.
This was causing opening the browser to fail because of 8243ff8bc8.
Before this change, renaming and deleting of open files (which can
easily happen due to the asynchronous nature of file systems) would
produce an error, for example saving files with Firefox.
After this change we open files with the flags necessary for open
files to be renamed or deleted.
Fixes#2730
