bonjour compat API needs more love

[lennart]

Quoting a mail form JP Roseavear:

Attached is a patch to improve mDNSResponder compatability.  For
DNSServiceBrowse and DNSServiceBrowser the apple lib returns an error if
ret_sdref is NULL and always guarantees the ret_sdref does not return
garbage (worst case it sets it to NULL).  Apps definitely rely on the
latter, see https://bugzilla.novell.com/show_bug.cgi?id=213891 for
gphoto2 at least.  Registering a service with an empty regtype should
also fail gracefully.

I remove the callback assert because apple allows that as well (maybe
not so good for avahi?)

[lathiat]

lennart - any reason not to apply this?

[lennart]

Looks fine to me.

[lathiat]

Fixed in R1345