Refactor: libstonithd: functionize fudging metadata of RHCS-style agent to make specific parameter non-required

The parameter port of RHCS-style fence-agents is fudged to be shown as
required="0" in the metadata output by stonith_admin --metadata or
crm_resource --show-metadata. So that users know they don't have to
configure it. Pacemaker handles and passes the parameter to the agent
when executing it.