[krbdev.mit.edu #7135] gssapi mechanism glue dlcloses objects potentially after they are already unloaded

[Greg Hudson via RT]

We decided to start using RTLD_NODELETE when loading plugin modules, 
mostly for OpenSSL-related reasons.  I think that should fix this 
problem; I would be interested in test results.

Of course, this change also makes it even more pointless to dlclose() GSS 
modules from the libgssapi_krb5 finalizer.  But we need to at least free 
the struct plugin_file_handle to avoid a memory leak on repeated loading 
and unloading of libgssapi_krb5, and there is also Windows to consider.