- Add support for CUPS printer instances

Don't just always use the `cups_dest_t`'s `name` for the printer

name, but also take it's `instance` member into account and if

present, append that for the name used for the CPDB printer name,

separated by a slash character (Pull request #34).

- Always query current CUPS default printer

When asked for the default printer, always query and return the

current CUPS default printer instead of whatever was the default

last time this was done, to take into account that the CUPS default

printer can change while the backend is running (Pull request #33).

- Pass correct parameters to `cupsStartDestDocument()`

Now job title and job attributes (options) are correctly passed on

(Pull request #36).

- Use NULL Instead of "NA" if there's no default printer

NULL makes clear that there's no default printer, while "NA" could

even be the name of an actual printer, so use the former instead of

the latter if no default printer could be determined (Pull request

#35).

- Use `g_strdup` instead of `cpdbGetStringCopy`

GLib's `g_strdup` already provides the same functionality as

`cpdbGetStringCopy` from cpdb-libs, so use the former instead of

relying on a custom CPDB implementation (Pull request #32).

AC_INIT([cpdb-backend-cups], [2.0b7], [https://github.com/OpenPrinting/cpdb-backend-cups/issues/], [cpdb-backend-cups], [https://github.com/OpenPrinting/cpdb-backend-cups])