Use krb5int_open_plugin for PKCS#11 module

Use krb5int_open_plugin for PKCS#11 module

Instead of calling dlopen() directly, use the krb5 cross-platform
interfaces (krb5int_open_plugin()).

The goal here is to eventually support pkinit on Windows; this is just
the first small step in that direction.

[ghudson@mit.edu: fixed memory leak; changed type of p11_module field;
added intermediate sym variable for strict aliasing conformance;
simplified out pkinit_C_UnloadModule() wrapper]