Open file/dirs once and use fildes-based APIs instead of path-based APIs

[enkore]

In line with #906, #908 and related issues we should try to use file descriptor / handle based operations in favour of path-based operations to avoid various race conditions and inconsistencies when files are replaced during a backup run.

These changes should go into master.

Windows can then use the _get_osfhandle API to retrieve the object backing a file descriptor of the CRTs POSIX-like API (open etc). And that can be used with the usual object-oriented Windows functions (e.g. GetUserObjectSecurity instead of GetFileSecurity).

[ThomasWaldmann]

Some progress on this has been made in #3918, #3960, #3961.

The gap between lstat(path) and fd = open(path) is still present, though.

[ThomasWaldmann]

note: we use os.scandir(path) and it requires python >= 3.7 to support giving a directory FD instead of a path on UNIX.

see also: benhoyt/scandir#108

[ThomasWaldmann]

fixed by #4043.