[Samba] Improve warning message about netbios name

Summary:
Before:
kf5.kio.core: We got some errors while running testparm "Load smb config files from /etc/samba/smb.conf\nLoaded services file OK.\nWARNING: The 'netbios name' is too long (max. 15 chars).\n\n"

After:
kf5.kio.core: Your samba 'netbios name' parameter was longer than the authorized 15 characters.
It may be because your hostname is longer than 13 and samba default 'netbios name' defaults to 'hostname-W', here: "meven-XPS-13-9370-W"
If that it is the case simply define a 'netbios name' parameter in /etc/samba/smb.conf at most 15 characters long

Reviewers: Frameworks, ngraham

Reviewed By: ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25355