kerberos: Fix numerous segfaults when using weak crypto

Weak crypto is provided by the openssl legacy provider which is
not load by default. Load the legacy providers as needed.

When the legacy provider is loaded into the default context the default
provider will no longer be automatically loaded. Without the default
provider the various kerberos applicaions and functions will abort().

PR: 272835
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D43009
Tested by: netchild, Joerg Pulz <Joerg.Pulz@frm2.tum.de>