[ upstream commit 542f0c2 ]

Write contents of the header file to a temporary file first. It will
then be atomically renamed to the real file. This makes sure we never
end up with corrupted on inconsistent header files on the filesystem.
Also make sure the symlink to the old header file in the downgrade case
is created atomically. The github.com/google/renameio package is used
for the atomic replace and symlink creation.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>