allow net ads join accept new osServicePack parameter

allow net ads join accept new osServicePack parameter

osServicePack paramater allows the default behaviour ( which is to use
the samba version string as the operatingSystemServicePack attribute )
to be overridden
Additionally make sure if blank string is passed that it is treated
as attribute deletion. This is necessary as values for the os attributes
are eventually passed to ads_modlist_add if the value is "" then the
attempt to add this attribute fails in the underlying ldap
'ldap_modfiy_ext_s' function. In this case we need to pass NULL as the
value to force deletion of the ldap attribute

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Thu Jan  8 00:18:05 CET 2015 on sn-devel-104