lib:cmdline: Check if we have a valid default ccache

lib:cmdline: Check if we have a valid default ccache

If you don't specify anything, and we have a valid ccache then try to
use it!

> smbclient -L //samba1.earth.milkyway.site
Anonymous login successful

        Sharename       Type      Comment
        ---------       ----      -------
        print$          Disk      Printer Drivers
        IPC$            IPC       IPC Service (Samba 4.22.1)
SMB1 disabled -- no workgroup available

In case the user specifies a principal, it will ask for a password:

> bin/smbclient -L //samba1.earth.milkyway.site -Ualice1@EARTH.MILKYWAY.SITE
Password for [alice1@EARTH.MILKYWAY.SITE]:

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>