s4:kerberos Add 'net export keytab' command for wireshark decryption

s4:kerberos Add 'net export keytab' command for wireshark decryption

It is much easier to do decryption with wireshark when the keytab is
available for every host in the domain.  Running 'net export keytab
<keytab name>' will export the current (as pointed to by the supplied
smb.conf) local Samba4 doamin.

(This uses Heimdal's 'hdb' keytab and then the existing hdb-samba4,
and so has a good chance of keeping working in the long term).

Andrew Bartlett