good work, but before we discuss merging this let's first talk about your usecase, and whether that usecase could be handled automatically in a sane way. whenever possible we should try to prefer sane defaults over additional config items.

For our devices, the IPv6 traffic path is not supported yet.
So, when the IPv6 addresses in the DNS response were returned to TinyProxy and when the IPv6 addresses were in the beginning of the DNS response,
TinyProxy would have to try these IPv6 addresses one after another consequently cause unnecessary TimeOut.

From the users' perspective, there would be obvious network performance issues.
We think that, this scenario might not be specific to us, to some extend, it's a general problem.
So, we'd like to add the general option to exclude either IPv6 or IPv4.

For our devices, the IPv6 traffic path is not supported yet.

this appears to be more of a configuration issue than anything else. the canonical solution to that would appear to be

echo "1" > /proc/sys/net/ipv6/conf/all/disable_ipv6
# disable automatic processing of router advertisement.
# this can silently and unexpectedly add ipv6 routes behind one's back.
echo "0" > /proc/sys/net/ipv6/conf/all/accept_ra

if the system is configured like this, the DNS resolver will not return ipv6 results.

as far as automatically selecting the best DNS reponse is concerned, we could just always request DNS results with AF_UNSPEC and then iterate over the results once trying to find an address that matches the address family that was specified for the listen address (or eventually bind addresses), and if none matching were encountered, just use the first response record. how and whether to prefer listen or bind addresses for this strategy would need some discussion.