Share handling of lucid_sec_context for Heimdal and MIT

Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
The 0.8 release of Heimdal has (will have) support for the lucid context.
The handling of lucid_sec_context can be shared between builds with MIT
or Heimdal Kerberos.

Split out the lucid_sec_context code from context_mit.c
and make a new common file, context_lucid.c.
Signed-off-by: Neil Brown <neilb@suse.de>