The DHCP plugins are linked statically and don't have a plugin mechanism
to be loaded from a shared library. Change the _nm_dhcp_client_register()
mechanism to something more static.

Also, only link the plugins into the binary if they are actually
enabled. Previously, they would always be linked in (and always
register themself). However, nm_dhcp_dh*_get_path() would return NULL
which made the plugin unusable.

The autoconf code to detect the DHCP plugins is still not stellar, but
seems to work well enough for now. At least, we log the result of the
detection at the end of the configure-script, so a user can at least
notice what happend.