multi: add minimum disk space check

sample-lnd.conf

@@ -418,3 +418,21 @@ litecoin.node=ltcd
 ; value must be >= 1m.
 ; healthcheck.chainbackend.interval=1m
 
+; The minimum ratio of free disk space to total capacity that we require.
+; healthcheck.diskspace.diskrequired=0.1
+
+; The number of times we should attempt to query our available disk space before
+; gracefully shutting down. Set this value to 0 to disable this health check.
+; healthcheck.diskspace.attempts=2
+
+; The amount of time we allow a query for our available disk space to take
+; before we fail the attempt. This value must be >= 1s.
+; healthcheck.diskspace.timeout=5s
+
+; The amount of time we should backoff between failed attempts to query
+; available disk space. This value must be >= 1s.
+; healthcheck.diskspace.backoff=1m
+
+; The amount of time we should wait between disk space health checks. This
+; value must be >= 1m.
+; healthcheck.diskspace.interval=6h