Bug 9497 - Make it easier to add new encodings for Z39.50 servers and add ISO 8859-1

Bug 9497 - Make it easier to add new encodings for Z39.50 servers and add ISO 8859-1

This patch does three things:
- Makes admin/z3950servers.pl create one variable for encoding,
  not one variable for each encoding
- Makes the template create encoding options from a list
- Adds ISO 8859-1 to the list of available encodings
See the bug for a lengthier description

To test:
- Apply the patch
- Edit one Z39.50 server several times, choosing each available
  encoding in turn
- Check that for each encoding, the correct encoding is shown both in
  the list of Z39.50 servers and in the dropdown on the edit screen

Comment: Works as described, no errors.

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Signed-off-by: Mason James <mtj@kohaaloha.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>