ulogd: load all plugins by default

ulogd: load all plugins by default

This new configuration behaviour option eases a bit the configuration of ulogd2
by allowing to load all plugins in one go, without having to know their full
path.

Choosing concrete plugins and using full path for them is great for some
environmnets, but I don't think it's a common case. The common case is to
load all plugins, even ignoring where do they live in the filesystem.

Even worse, the full path may be architecture-dependant, which makes copying
the ulogd.conf file between machines unnecesarily complex.

To experiment this new behaviour, don't put any 'plugin=' directive in the
config file. Plugins will be loaded from a default directory, choosen at
build/configure time (--with-ulogd2libdir). If no specified, this is something
like '/usr/local/lib/ulogd/'.

This new configuration option doesn't implement any special logic. We simply
open the dir and try to load all files ending with '.so'.

The log message level for plugins loading is increased so users can see by
default which plugins are loaded.

Signed-off-by: Arturo Borrero Gonzalez <arturo@netfilter.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>