* plugin.c (dlerror): New. Defined if HAVE_DLFCN_H isn't
defined.
(plugin_opt_plugin): Change return type to void. Stop on
dlopen error and report error with dlerror ().
(plugin_load_plugins): Change return type to void. Stop on
dlsym error and report error with dlerror (). Don't use
set_plugin_error.
(plugin_call_cleanup): Issue an error for each plugin.
* plugin.h (plugin_opt_plugin): Change return type to void.
(plugin_load_plugins): Likewise.