dnssd: Prevent crash when resolving service
dnssd: Prevent crash when resolving service

AvahiClient appears to require that avahi_service_resolver_new is
invoked from the same thread to which its poll function is bound
otherwise it can crash with a callback running while
avahi_service_resolver_new is still busy.

To fix this, always run avahi_service_resolver_new from the main loop.
To simplify the code, any errors from the function are ignored for now.

This crash could be reproduced 100% of the time by trying to mount
dav+sd (via gnome-user-share) in a single-CPU VM.

https://bugzilla.gnome.org/show_bug.cgi?id=629345

gicmo/gvfs,gicmo/gvfs,gicmo/gvfs,gicmo/gvfs