const, the errors framework, and -Wcast-qual

[Greg Hudson]

To close out this thread:

* I removed -Wcast-qual from the standard gcc warnings set.

* I'm not going to unconstify the errors framework further, because I
realized that it would be pointless to change krb5_get_error_message
without also changing krb5_free_error_message, and changing the latter
to take a char * would be incompatible with existing code.