[krbdev.mit.edu #3971] broken configure test for dlopen

[Ken Raeburn via RT]

Currently we look for dlopen() in -ldl.  We *don't* look to see if we
get dlopen without that library.  So on a system without that library,
like Tru64 5.1, the plugin support (including all KDC-side database
access) will be broken.

The failure mode for not having a plugin system implemented is to
quietly not find any plugins, and let the caller deal (or fail to).
For example, from kdb5_util create:

create: Unable to find requested database type while creating database '/home/raeburn/krb5/krb5-1.5/alpha/Install/var/krb5kdc/principal'


Ken