systemd/systemd-services: Add additional hexagonrpcd unit files, V2

Clayton Craftrequested to merge
craftyguy/hexagonrpc_systemd into master

New spin of !6105 (closed), which fixes hexagonrpcd on systemd

cc @rmader @rdacayan

Fixes #3447 (closed) and #3468 (closed)

Edited by Bart Ribbers

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