s3:libads: Allow 'net ads keytab add' handle Windows SPN(s) part 1

s3:libads: Allow 'net ads keytab add' handle Windows SPN(s) part 1

This patch addresses how the windows SPN is converted into a kerberos
priniciple to be written to the keytab file. A followup patch will
deal with writing Window SPN(s) to the AD.

Before this change 'net ads keytab add' handled three scenarios

a) 'net ads keytab add param' is passed a fully qualified kerberos principal
   (identified by the presence of '@' in param) In this scenario the keytab
   file alone is updated with the principal contained in 'param'.
b) 'net ads keytab add param'; is passed a machine name (identified by
   the paramater ending with '$'). In this case the machine name
   is converted to a kerberos principal with according to the recipe
   'param@realm' where realm is determined by lp_realm().
c) 'net ads keytab add param' is passed a service (e.g. nfs, http etc.)
   In this scenario the param containing the service is first converted to
   into 2 kerberos principals (long and short forms) according to the
   following recipe
      i) long form:  'param/fully_qualified_dns@realm'
     ii) short form: 'param/netbios_name@realm'
     where 'fully_qualified_dns is retrieved from 'dNSHostName' attribute of
     'this' machines computer account on the AD.
     The principals are written to the keytab file
   Secondly 2 windows SPNs are generated from 'param' as follows
      i) long form 'param/full_qualified_dns'
     ii) short form 'param/netbios_name'
   These SPNs are written to the AD computer account object

After this change a) & b) & c) will retain legacy behaviour except
in the case of c) where if the 'param' passed to c) is a Windows SPN
(e.g. conforming to format 'serviceclass/host:port'
  i) 'param' will get converted to a kerberos principal (just a single one)
     with the following recipe: 'serviceclass/host@realm' which will
     be written to the keytab file. The SPN written to the AD is created
     as before and the legacy behaviour is preserved.

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>