Small clean-up of config file syntax: group dbus related options together

Old-school:

  <dbus name="org.opensuse.Network" />
  <schema name="/some/path/wicked.xml" />

New school:

  <dbus>
    <service name="org.opensuse.Network" />
    <schema name="/some/path/wicked.xml" />
  </dbus>

For the time being, both old-school and new-school options
will be recognized. No warnings are issued.

Signed-off-by: Olaf Kirch <okir@suse.de>
config: support a local.xml to override settings w/o getting in trouble with maintenance updates

If a user modifies one of the wicked xml config files, we're running
into trouble on the next maintenance update. Either the poor guy/gal is
using all modifications, or we're not applying our change.

One way around that is to have a dangling <include> element in common.xml:

  <!-- If you want to override configuration options
       in this file, consider putting them into
       local.xml rather than here.
    -->
  <include name="local.xml" optional="true"/>

We don't ship that local.xml. If it doesn't exist, we don't complain,
but if the user does create it, we parse it like any other config file.

Signed-off-by: Olaf Kirch <okir@suse.de>
Minor cosmetic error logging fix to ni_config_parse_addrconf_*

Several ni_error() calls were referring to an XML element by

   <something attribute="foobar"

without closing right angle. Fixed those.

Signed-off-by: Olaf Kirch <okir@suse.de>
Updated wicked-config.5 manpage

Signed-off-by: Olaf Kirch <okir@suse.de>
Merge pull request #411 from okirch/config-doc

Some config.xml enhancements and big manpage update