Merge pull request #111 from shivrai/feat/close-fin

Test for closing a connection using FIN flag

gtests/net/packetdrill/tests/bsd/tcp/close/close-fin.pkt

+// Test for checking if the client generates a FIN
+// after we close an established connection.
+
+0.000 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+0.000 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+0.000 bind(3, ..., ...) = 0
+0.000 listen(3, 1) = 0
+
+0.100 < S 0:0(0) win 65535 <mss 1460,sackOK,nop,nop,nop,wscale 7>
+0.100 > S. 0:0(0) ack 1 <...>
+0.200 < . 1:1(0) ack 1 win 65535
+0.200 accept(3, ..., ...) = 4
+
+// Client closes the connection
+0.300 < F. 1:1(0) ack 1 win 65535
+0.300 > . 1:1(0) ack 2 win 1026
+
+// We close the connection
+0.400 close(4) = 0
+0.400 > F. 1:1(0) ack 2 win 1026