Contrib/redhat updates

This is an update of #186, taking almost all feedback posted there in account. It also updates the specfile for 5.2.0 and is rebased on the collectd-5.2 branch.

NB: the perl bindings currently *won't* build, without 25fd24689 (from @tokkee's sh/perl-build branch)

@rubenk: many many thanks for your patches !

@faxm0dem: so now you should be able to build the rrdcached plugin with `rpmbuild -ba --with rrdcached`

@tokkee: I still plan to split out collection3 from -contrib into a dedicated package. no worry ;)