Merge bitcoin/bitcoin#35978: contrib/init: fix unused variables in openrc script

d837bb38a44de445d2c459a477302d78871ae1a6 contrib/init: fix unused variables in openrc script (jpk68)

Pull request description:

  - Makes it so that `${BITCOIND_BIN}` is actually used as `command=`, rather than the hardcoded `/usr/bin/bitcoind`.
  - Passes `BITCOIND_GROUP` to `start-stop-daemon`, so that the daemon process itself runs under it.

ACKs for top commit:
  jeanpablojp:
    utACK d837bb38a44de445d2c459a477302d78871ae1a6
  thomasbuilds:
    ACK d837bb38
  winterrdog:
    utACK d837bb38a44de445d2c459a477302d78871ae1a6

Tree-SHA512: 78c237224d65cc47ade606df4808fbf4ca70109d95301c35d1b336eead1b25138a83ce6f2314775b8a11a6745af2220af9027640530b9edb7e0167ccea081b6b