extensions: libxt_socket: update man pages and tests for --restore-skmark

Update the man pages for libxt_socket with a description and example
usage of the --restore-skmark option.

Also added tests for libxt_socket with various combinations of
--restore-skmark and the existing options.

Signed-off-by: Harout Hedeshian <harouth@codeaurora.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
diff --git a/extensions/libxt_socket.t b/extensions/libxt_socket.t
index 8c0036e..fe4eb3e 100644
--- a/extensions/libxt_socket.t
+++ b/extensions/libxt_socket.t
@@ -2,3 +2,7 @@
 *mangle
 -m socket;=;OK
 -m socket --transparent --nowildcard;=;OK
+-m socket --transparent --nowildcard --restore-skmark;=;OK
+-m socket --transparent --restore-skmark;=;OK
+-m socket --nowildcard --restore-skmark;=;OK
+-m socket --restore-skmark;=;OK