This is only used in TempPathFactory.getbasetemp(). We'll be wanting
further control/care there, so move it into there.

…ions

(Written for a Unix system, but might be applicable to Windows as well).

pytest creates a root temporary directory under /tmp, named
`pytest-of-<username>`, and creates tmp_path's and other under it.
/tmp is shared between all users of the system.

This root temporary directory was created with 0o777&~umask permissions,
which usually becomes 0o755, meaning any user in the system could list
and read the files, which is undesirable.

Use 0o700 permissions instead. Also for subdirectories, because the root
dir is adjustable.

pytest uses a root temp directory named `/tmp/pytest-of-<username>`. The
name is predictable, and the directory might already exists from a
previous run, so that's allowed.

This makes it possible for my_user to pre-create
`/tmp/pytest-of-another_user`, thus giving my_user control of
another_user's tempdir.

Prevent this scenario by adding a couple of safety checks. I believe
they are sufficient.

Testing the first check requires changing the owner, which requires
root permissions, so can't be unit-tested easily, but I checked it
manually.