s3-util: Compare the maximum allowed length of a NetBIOS name

s3-util: Compare the maximum allowed length of a NetBIOS name

This fixes a problem where is_myname() returns true if one of our names
is a substring of the specified name.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11427

Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
(cherry picked from commit 4e178ed498c594ffcd5592d0b792d47b064b9586)

Autobuild-User(v4-2-test): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(v4-2-test): Mon Aug 31 12:33:42 CEST 2015 on sn-devel-104