RADIUS client: Re-try connection if socket is closed on retransmit

RADIUS client: Re-try connection if socket is closed on retransmit

Previously, send() was called with invalid fd = -1 in some error cases
for retransmission and this could even result in a loop of multiple such
attempts. This is obviously not going to work, so drop such attempts and
instead, try to reconnect a socket to the server if the current socket
is not valid.

In addition, initiate server failover immediately if the current socket
is not valid instead of waiting for a timeout.

Signed-off-by: Jouni Malinen <j@w1.fi>