libmemcached: set meta.platforms

And a more precise license (BSD => 3-clause BSD).
collectd: enable a bunch of plugins (by adding more deps)

These are plugins that are internal to collectd; they are bundled in the
source archive. Without this, many of the collectd plugins advertised on
their homepage don't actually work, so I think we should add this.

This change increases the closure size from 129 MiB to 614 MiB. The size
of the collectd store path itself goes from 2 MiB to 2.8 MiB.

Users wanting a smaller closure size can easily set some (or all) of the
optional inputs to "null" using ".override".
collectd: update meta attributes

Don't repeat package name in description, add maintainer, ...