Bug 9032: (follow-up) Plackify opac-shareshelf.pl

Bug 9032: (follow-up) Plackify opac-shareshelf.pl

Plackified opac-shareshelf by moving global my variables into hashref
and explicitly passing paramters to the subroutines that
opac-shareshelf.pl uses.

Slightly changed base64ord and base64chr functions to more generic ones.

This patch also corrects the version of the GPL statement.

Test plan:
Verify if sending an share invitation still works (under prog theme).

Signed-off-by: Dobrica Pavlinusic <dpavlin@rot13.org>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>