Protect CFBundle calls with mutexes

Protect CFBundle calls with mutexes

CFBundles are refcounted and the recounts are not threadsafe.
Protect CFBundles used for loading bundled plugins with a
mutex to prevent crashes when multiple threads are loading
and unloading the same plugin.

As part of this we use thread-safe dlopen/dlsym/dlclose
for the actual loading and unloading and just use CFBundle
to get the path to the actual executable.  This reduces
the number of places we need to wrap CFBundles with mutexes
and the amount of Mac-specific code in the plugin code.

ticket: new

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@20285 dc483132-0cff-0310-8789-dd5450dbe970