server/sockets.c: Don't fail to start if ipv6.disable=1

If ipv6.disable=1 is used on the Linux kernel command line,
getaddrinfo still returns AF_INET6 sockets but the subsequent
socket(2) call fails with EAFNOSUPPORT:

  $ cat /proc/cmdline
  BOOT_IMAGE=(hd0,gpt2)/vmlinuz-5.17.13-300.fc36.x86_64 root=UUID=0c643cb4-703c-4625-9ba8-93e208b9b827 ro console=tty0 rd_NO_PLYMOUTH console=ttyS0,115200 ipv6.disable=1 pcie_aspm=off net.ifnames=0

  $ nbdkit -p 10000 null
  bind_tcpip_socket: socket: Address family not supported by protocol

I believe it's safe to ignore this case, as long as we manage to open
at least one socket (eg. AF_INET).

Reported-by: Bogac Tekman
Fixes: https://aur.archlinux.org/packages/nbdkit#comment-877225

Author: rwmjones

server/sockets.c

@@ -209,8 +209,20 @@ bind_tcpip_socket (sockets *socks)
     sock = set_cloexec (socket (a->ai_family, a->ai_socktype, a->ai_protocol));
 #endif
     if (sock == -1) {
-      perror ("bind_tcpip_socket: socket");
-      exit (EXIT_FAILURE);
+      if (errno == EAFNOSUPPORT) {
+        /* If ipv6.disable=1 was specified to the Linux kernel then
+         * getaddrinfo may still return AF_INET6 sockets but socket(2)
+         * will return this error.  I think it's safe to basically
+         * ignore this error.
+         */
+        saved_errno = errno;
+        debug ("bind_tcpip_socket: socket: %m (ignored)");
+        continue;
+      }
+      else {
+        perror ("bind_tcpip_socket: socket");
+        exit (EXIT_FAILURE);
+      }
     }
 
     opt = 1;