Change the default plugin install path and name

Change the default plugin install path and name

The default installation path for this plugin is now
/usr/local/lib/krb5/plugins/pwqual/strength.so (for both MIT and
Heimdal), assuming a --libdir setting of /usr/local/lib.  This may
require updates to the Kerberos KDC configuration or moving the plugin
when upgrading from earlier versions.