Munin contributed plugins

Fix spelling issues

smart: importing common.{h,c} from munin-c

The compilation is very easy, a simple `make` should work, as I provided
a convenient `Makefile`

Hacked together a smart_ plugin in C

There is some trickery going on to not wakeup the disk when it's in standby

Note: this was aimed at munin-c, but was rejected since it uses a
subprocess that calls the `smartctl` tool.