s3:rpcclient: make use of SMB_SIGNING_IPC_DEFAULT

s3:rpcclient: make use of SMB_SIGNING_IPC_DEFAULT

This means we'll use the "client ipc min protocol", "client ipc max protocol"
and "client ipc signing" options. But "--signing=no" or "--signing=required"
still overwrite "client ipc signing".

The following can be used to alter the max protocol

rpcclient --option="client ipc max protocol=SMB2_10" 172.31.9.163 -Uadministrator%A1b2C3d4 -c "getusername"
Account Name: Administrator, Authority Name: W4EDOM-L4

rpcclient --option="client ipc max protocol=NT1" 172.31.9.163 -Uadministrator%A1b2C3d4 -c "getusername"
Account Name: Administrator, Authority Name: W4EDOM-L4

rpcclient 172.31.9.163 -Uadministrator%A1b2C3d4 -c "getusername"
Account Name: Administrator, Authority Name: W4EDOM-L4

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11927

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Sat May 21 05:01:15 CEST 2016 on sn-devel-144

(cherry picked from commit 2eb824fbaf61dfc5e9c735589c80c41379dabe86)

Autobuild-User(v4-2-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-2-test): Mon May 30 13:55:41 CEST 2016 on sn-devel-104