Do not fence connections without valid greeting.

There is no reason to fence any connection that hasn't sent a valid
greeting, since they haven't progressed far enough for it to make
sense.

We remove the connection from the list of accepted clients, which then
removes the need for fencing, and the server won't need to restart.

Adds a test script that makes sure that we didn't actually restart the
server while this was happening.

Signed-off-by: Auke Kok <auke.kok@versity.com>

tests/sequence

@@ -65,4 +65,5 @@ block-stale-reads.sh
 freed-list-wedge.sh
 inode-deletion.sh
 renameat2-noreplace.sh
+portscan.sh
 xfstests.sh