tree-wide: ignore messages with too long control data

Apparently SELinux inserts control data into AF_UNIX datagrams where we
don't expect it, thus miscalculating the control data. This looks like
something to fix in SELinux, but we still should handle this gracefully
and just drop the offending datagram and continue.

recvmsg_safe() actually already drops the datagram, it's just a matter
of actually ignoring EXFULL (which it generates if control data is too
large) in the right places.

This does this wherever an AF_UNIX/SOCK_DGRAM socket is used with
recvmsg_safe() that is not just internal communication.

Fixes: #17795
Follow-up for: 3691bcf3c5

src/shared/ask-password-api.c

@@ -943,6 +943,10 @@ int ask_password_agent(
                 n = recvmsg_safe(socket_fd, &msghdr, 0);
                 if (IN_SET(n, -EAGAIN, -EINTR))
                         continue;
+                if (n == -EXFULL) {
+                        log_debug("Got message with truncated control data, ignoring.");
+                        continue;
+                }
                 if (n < 0) {
                         r = (int) n;
                         goto finish;