init_context() currently just returns a null pointer if the C routine
fails, which is unlike the norm in this module and I assume is an
oversight. Typically this will fail if there's a syntax error in the
libkrb5 configuration (/etc/krb5.conf, or whatever is read). A program
I wrote segfaulted in this situation, because I then passed the null
context to other routines. With this change, we get instead:

In [2]: ctx = krb5.init_context()
---------------------------------------------------------------------------
Krb5Error                                 Traceback (most recent call last)
...
Krb5Error: Included profile file could not be read -1429577697

This is part of the MIT Kerberos "localauth" interface:

  https://web.mit.edu/kerberos/krb5-devel/doc/plugindev/localauth.html

... so I used that as the "extension" grouping name. There is also a
krb5_kuserok() in this interface, that might be added.

I'm moving this to its own PR; it's unrelated to the
krb5_aname_to_localname() addition.

This reverts commit cda1e05.