Add BITCOIND_SIGTERM_TIMEOUT to OpenRC init scripts

[Flowdalic]

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.

[luke-jr]

utACK

[laanwj]

utACK

[paveljanik]

utACK