Initial enhanced error message support, similar to what I sent to

Initial enhanced error message support, similar to what I sent to
krbdev except for some function renaming (krb5_free_error was already
in use, so added _message to everything), and the context is allowed
to be NULL (in which case we fall back to error_message() and storing
no strings) to simplify some code.

Low-level routines in the support library, using a private data
structure; higher-level routines in libkrb5, using a krb5_context.

Added error info strings to the KRB_ERR_GENERIC case in gc_via_tkt.c
and the python sample service location plugin.  Added code to kinit
and kvno to look up and display the strings.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@17776 dc483132-0cff-0310-8789-dd5450dbe970