socket-default: Don't set SO_REUSEADDR on IKE sockets anymore

socket-default: Don't set SO_REUSEADDR on IKE sockets anymore

This was originally required when pluto and charon both bound sockets to
the same port to send messages.  Pluto also received messages on them but
charon didn't and used a raw socket instead.  Since the removal of pluto
we don't need to set this option anymore, which might actually mask
mistakes like running charon and charon-systemd concurrently (that could
result in messages getting sent fine by both daemons but only received
by one).

Note that a failure to create/bind the sockets will not immediately
result in a shutdown of the daemon.  Instead, there will be an error
once the receiver tries to read any messages and also whenever the sender
attempts to send a request.