Enhancement: Support UTF-16 encoding and process Byte Order Mark

On Windows, text editors commonly leave a Byte Order Mark at the beginning of
the file to define which encoding is used (oftentimes UTF-16). We've added code
to support processing the BOMs in text files, like the exclude files, the
password file and the file passed via `--files-from`. This does not apply to
any file being saved in a backup, those are not touched and archived as they
are.

https://github.com/restic/restic/issues/1433
https://github.com/restic/restic/issues/1738
https://github.com/restic/restic/pull/1748
