Merge from plugin branch

Merge from plugin branch

Add plugin support:
- plugin routines in support library (may break windows build!)
- plugin support in KDC location code
- sample Python-based plugin for KDC location, not built without
  tweaking sources
- changed service location interface to use an enum instead of passing
  profile string and DNS strings and port numbers
- changed pathnames for plugin locations, including kdb back end
- remove locate_service from accessor API

Also, do build shared libraries for Darwin just like any other UNIX box.

Not present yet:
- use new plugin interface for kdb back end
- Windows support
- Mac bundle support (but dlopen support works)
- search path for libkrb5 plugins (only one hard-coded directory for now)
- sorting of plugin collections for predictable ordering

See the various ChangeLogs for specifics.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@17706 dc483132-0cff-0310-8789-dd5450dbe970