Allow printer-dns-sd-name to be set

[michaelrsweet]

Currently cupsd doesn't allow the user/admin to change the printer-dns-sd-name value. We want to change that (already supported for CUPS 3.0 via PAPPL):

• Load/save printer-dns-sd-name (reg_name) values in printers.conf and classes.conf
• Update add_class, add_printer, and set_printer_attrs to allow "printer-dns-sd-name" to be changed
• Make sure that "printer-dns-sd-name" is listed in cups/encode.c
• Document setting values in the lpadmin man page
• Update the classes.conf and printers.conf man pages
• Update admin.cgi and the templates to provide a text field for setting/changing the DNS-SD name (up to 63 bytes).