contrib/init: (OpenRC) use -startupnotify to wait for startup completion [alternative]

This is an alternative to #22285 to address @luke-jr's concern.

When other initscripts depend on bitcoind, it's because their daemons want to be able to invoke bitcoin-cli or to communicate with bitcoind via RPC. That can't happen until some time after bitcoind has forked into the background. The -startupnotify option was added in 090530cc (#15367) to support exactly this use case, so let's use it.

A straightforward and reliable way to wait for bitcoind to invoke its -startupnotify command is to make bitcoind hold a lock on a dummy file and release that lock via the -startupnotify command. The initscript's start_post() function initially marks the service as inactive and forks a subshell process into the background to wait for bitcoind to release the lock file, which will happen when bitcoind either executes the -startupnotify command or dies. After acquiring and releasing the lock, the subshell checks whether the pid file still exists and marks the service as started or stopped appropriately.