lib/krb5: krb5_get_instance does not work on Windows 7

lib/krb5: krb5_get_instance does not work on Windows 7

krb5_get_instance() is meant to ensure that the shared library
instance of heimdal loaded by a plugin matches the instance that
loaded the plugin.  It works by declaring a static C string whose
memory address will be used as an instance identifier.  If the
instance returned from the plugin matches the instance obtain
by the code that loads the plugin, then we can conclude the two
instances are the same.

This doesn't work on Windows 7.  When heimdal.dll loads a plugin
that is linked to heimdal.dll, the plugin's heimdal.dll is always
a new instance.  However, the requirement for plugin safety is
not that the plugin be the same instance in memory but that they
be the same instance on disk.

This change loads the path name and version string for the module
and generates a hash of those strings as an instance identifier.

Change-Id: I1c0651969e9738c5feecb0b323969d13efd4704d