Bug 7250: stage_biblios_file.pl is missing options for encoding

Bug 7250: stage_biblios_file.pl is missing options for encoding

- adds command line options for encoding, defaulting to utf8
- options match options availablen in the stage marc records form
  of the staff interface
- activates warnings
- adds copyright statement

To test:
Import records with diacritics using the stage_biblios_file.pl
Records can be imported into the catalog using the staff interface
or the commit_biblios_file.pl script.

Signed-off-by: Ulrich Kleiber <ulrich.kleiber@bsz-bw.de>
Successfully tested with default encoding utf8.

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>