Default client keytab name

[Nico Williams]

I prefer consistency, so I prefer UIDs for keytabs because we already
use that for ccaches.  That said, I won't mind if there's a token that
means "username" and one that means "uid" and then we can leave it up
to the vendors and sysadmins which to use.

Also, it's not /etc that worries me but nscd.  In self-credentialed
mode nscd would deadlock if getpwuid() were needed.