Set an error message when keyring get_princ fails

When attempting to use a keyring cache that doesn't exist, set an error
message when we fail to read a principal name, as we do when we return
the same error code when using a file ccache.

[ghudson: removed unnecessary check for d->name nullity.]

ticket: 7809
target_version: 1.12.1
tags: pullup
Use an extended com_err hook in klist

Add an adapted version of extended_com_err_fn from kinit to klist and
use it.  In do_ccache(), rely on the ccache type to set a reasonable
message if krb5_cc_set_flags() or krb5_cc_get_principal() fails due to
a nonexistent or unreadable ccache, and don't confuse the user with
the name of the ccache operation that failed.

ticket: 7809
Avoid keyctl purge in keyring ccache tests

keyctl purge was added in keyutils 1.5 (released in March 2011).  Use
keyctl unlink to clean up keys instead, as it is more universal.

ticket: 7810
target_version: 1.12.1
tags: pullup
Require built-in verto for make depend

A tree configured to use the system libverto will be missing
$(VERTO_DEPS) in dependencies, so disallow make depend.