Two munin issues (colour handling and agent-side environment setup)

Issue 1: The observium-side munin script uses inconsistent names for color in the $ds array – polling/unix-agent/munin-plugins.inc.php sets $ds['ds_colour'], but the html/includes/graphs/munin/graph.inc.php is looking for $ds['colour'].

Issue 2: The MUNIN_LIBDIR environment value is not set by the agent-local/munin script, and many munin scripts look for it.

Patches for both are attached.