freeipmi: implement suggestions from Bart ten Brinke for FreeIPMI 1.1.x.

This adds support for versions of FreeIPMI before 1.2 (without the
threshold information), and explicits the dependency information.

It also works with older Munin nodes where dirtyconfig is not supported.
hwmon: support non-dirtyconfig capable nodes.
asterisk: first part of the new Asterisk plugin.

This plugin is targeting to enter main distribution in 2.1 to remove
the multiple Asterisk plugins present in 2.0.

It features full autoconfiguration support, multigraph capabilities,
and less error-prone code.

For the moment, it only implements a replacement for asterisk_channels.
asterisk: add support for showing the per-type breakdown of channels.

This makes it possible for a single graph to replace the two
apache_channels and apache_channeltypes plugins.
asterisk: add asterisk_voicemail replacement code as well.

This is a bit trickier, since the voicemail might not be enabled
anywhere. Unfortunately it's not easy to set this up during
autoconfig, as voicemails can be added on the fly.
asterisk: handle more gracefully errors on the asterisk_channels path.
asterisk: refactor command/response handling.

Instead of having a dumb readreply, already drop some of the unneeded
lines of the command when sending the request, and handle errors in
place.

This simplifies the code a little, even though it requires a few more
"discard loops" in the code (to avoid garbage in Asterisk's logs).
asterisk: add graph to replace asterisk_meetme and asterisk_meetmeusers.
asterisk: replace the remaining two Asterisk plugins.

The fourth graph (asterisk_codecs) replaces both asterisk_sipchannels
and asterisk_codecs, as the latter already included the data from the
first. If we want to get more details we could have sub-multigraphs
for a breakdown of the codecs per channel type, but right now it feels
unneeded.
Fixed undefined warnings
Merge pull request #231 from barttenbrinke/master

Fix freeimpi undefined warnings