fopen: create new file using old file's mode

fopen: create new file using old file's mode

Because the function renames the temp file to the target name as a last
step, if the file was previously owned by a different user, not ORing
the old mode could otherwise end up creating a file that was no longer
readable by the original owner after save.

Reported-by: Loïc Yhuel
Fixes #12299
Closes #12395