Load plugins with RTLD_NODELETE if possible

On platforms which support RTLD_NODELETE, use it to load plugin
modules.  While using this flag makes plugins stay in the process map
after libkrb5/libgssapi_krb5 are unloaded, it solves several problems:

1. It prevents plugin modules which link against OpenSSL (PKINIT and
k5tls) from repeatedly initializing instances of libssl or libcrypto,
leaking heap memory each time.  This is only an issue because we
cannot safely uninitialize OpenSSL.

2. It prevents finalization ordering issues from causing a process
crash when unloading libgssapi_krb5 (issue #7135).

3. It makes memory leak tracing with valgrind easier.