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>
2 files changed