bug#17976: 24.3; url-retrieve-synchronously doesn't fallback to IPv4

[Juliusz Chroboczek]

Here's a trace (edited to remove irrelevant stuff).  I'm only guessing,
but to my untrained eyes it looks like the code around process.c:4920.

-- Juliusz

socket(PF_INET6, SOCK_STREAM, IPPROTO_TCP) = 13
fcntl(13, F_SETFL, O_RDONLY|O_NONBLOCK) = 0
connect(13, {sa_family=AF_INET6, sin6_port=htons(8080), inet_pton(AF_INET6, 
"2001:660:3301:8070::40", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, 28) = 
-1 EINPROGRESS (Operation now in progress)
getsockname(13, {sa_family=AF_INET6, sin6_port=htons(33826), 
inet_pton(AF_INET6, "2a01:e34:ec22:84a0::", &sin6_addr), sin6_flowinfo=0, 
sin6_scope_id=0}, [28]) = 0
fcntl(13, F_SETFL, O_RDONLY|O_NONBLOCK) = 0
pselect6(14, [12], [13], NULL, {1957, 16494001}, {NULL, 8}) = 1 (out [13], left 
{1957, 10004292})
getsockopt(13, SOL_SOCKET, SO_ERROR, [111], [4]) = 0
close(13)                               = 0