s3-smbd: use fruit:model string for mDNS registration

s3-smbd: use fruit:model string for mDNS registration

With this change we now allow to modify the icon to represent Samba in
Finder. Possible values are at least:

fruit:model = iMac
fruit:model = MacBook
fruit:model = MacPro
fruit:model = Xserve
fruit:model = RackMac

Prior to this change we only displayed the correct icon when a mac
client negotiated the apple create context over SMB.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13746

Based on proposed patch from Rouven WEILER <Rouven_Weiler@gmx.net>

Guenther

Signed-off-by: Guenther Deschner <gd@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Autobuild-User(master): Günther Deschner <gd@samba.org>
Autobuild-Date(master): Tue Jan 15 21:27:20 CET 2019 on sn-devel-144

(cherry picked from commit 538ce72f1b2fa78450e3b711e58bd0e238faf466)