filter_file: fix various bugs (#48038)

* `f.tell` on a `TextIOWrapper` does not return the offset in bytes, but
  an opaque integer that can only be used for `f.seek` on the same
  object. Spack assumes it's a byte offset.
* Do not open in a locale dependent way, but assume utf-8 (and allow
  users to override that)
* Use tempfile to generate a backup/temporary file in a safe way
* Comparison between None and str is valid and on purpose.