Add comment how further options can be added or existing ones
modified. Use /run/${RuntimeDirectory} for PID file.
Remove TimeoutStopSec, TimeoutStartSec, StartLimitInterval,
StartLimitBurst directives as those should be set indivdually.
Remove Group to user the bitcoin user's default group.
Changed Restart from 'always' to 'on-failure' (can also be overwritten
individually).
This allows users to specify, e.g. raise, the default timeout of 60
seconds. Some bitcoind instances, especially long running ones on slow
hardware, require a higher timeout for a clean shut down.
Also add a comment to bitcoind.openrc's 'retry=', since it is not
obvious from the variable name what it does.
