upnp: provide our own User-Agent and Server HTTP header values

Some people using PlayOn to access XBMC over UPnP have noticed that XBMC doesn't provide a User-Agent and/or Server HTTP header that makes it obvious that the server is XBMC. I've checked and the convention seems to be to use a value of the form
```
UPnP/1.0 DLNADOC/1.50 <application>[/<version>] [<operating-system/<operating-system version>]
```
Some also include the operating system but since we have to hardcode this value for Platinum SDK (and Neptune SDK) to pick this up I didn't include the operating system part or the version of XBMC.

I'll post test builds for the people who reported this issue to get them to confirm that this makes interaction with PlayOn (and probably other software) easier.