(maint) Move duplicate code in /lib/puppet/provider/service/freebsd.rb into single method

Prior to this commit, service_name and rcvar_name contained duplicate
code in lib/puppet/provider/service/freebsd.rb that accomplished the
same thing. This commit moves this code into a single method that
can be invoked from both service_name and rcvar_name.
Merge pull request #2723 from briancain/maint/master/consolidate-methods

(maint) Move duplicate code in /lib/puppet/provider/service/freebsd.rb i...