take $RIEMANN_OPTS in account in debian initscript

This makes it the same as redhat, which allows users to set
RIEMANN_OPTS="-a"
in /etc/{default,sysconfig}/riemann
mention more options in /etc/{sysconfig,default}/riemann
Merge pull request #332 from mfournier/riemann_opts_initscript

Riemann initscript minor improvements