another thread to the mainthread in a safe manner.
This is used for the DNS lookup code, and can also
be used by the signal handler to safely report actions back
to the application mainloop without using global variables.
Previously you would have to do something like this:
for (type foo = (type) list_get_first(list); foo; foo = (type) list_get_next(list)
{
/* code */
}
Now, you can instead write this as:
LIST_FOREACH(type, foo, list,
{
/* code */
})
Basically, boilerplate stuff including the casting is gone.
This API provides transparent asynchronous DNS lookups
with both IPv4 and IPv6 support, and in addition will try
to connect to all addresses until one of them work.
This implements the "happy eyeballs" algorithm provided that the client supports IPv6
and that the DNS records provides both IPv6 and IPv4 addresses.
