pyppd creates double entries for many (all ?) standard PPDs

Hi Vitor,

my use-case is the building of foomatic-db-compressed-ppds on Debian (similarly on Ubuntu). The end-result is the following:

```
$ /usr/lib/cups/driver/foomatic-db-compressed-ppds list | grep B6300-Postscript
"foomatic-db-compressed-ppds:0/ppd/foomatic-ppd/Oki-B6300-Postscript.ppd" en "Oki" "Oki B6300 Foomatic/Postscript (recommended)" "DRV:DPostscript,R1,M0,TP;"
"foomatic-db-compressed-ppds:1/ppd/foomatic-ppd/Oki-B6300-Postscript.ppd" en "Oki" "Oki B6300 Foomatic/Postscript (recommended)" "MFG:Oki;MDL:B6300;"
```

As you can see, this PPD file has two entries in the pyppd-compiled list. one DRV and one MFG/MDL pair.

This has the annoying side-effect to show these doubled entries in many (if not all) CUPS interfaces: localhost:631, system-config-printer-kde, etc.

What do you think, should these doubled entries for "single-printer, single-driver" be merged, and if so, how?

Cheers, OdyX