[krbdev.mit.edu #8420] built in plugins need a method to disable/enable by default

[Greg Hudson via RT]

We can have a configuration option to enable .k5identity which defaults to 
true in the upstream MIT krb5 and defaults to false in Solaris (via a 
configure option).  I would still prefer to handle this inside 
ccselect_k5identity.c rather than in the plugin infrastructure, since 
configuration within the plugin infrastructure would be much more arcane 
from the administrator's perspective.