Allow a msg value of "0" to clear/unset the node/+/set/# topics/flags

e.g. publishing "0" to node/#/set/intr will clear the node-interrupt flag.
Any non-zero message value will "set" the flag. Same is true for the
lock, down, and rerun topics/flags.

These node attributes (misc) flags could be cleared previously
by setting the 'misc' topic (e.g. to 0), but since that's not an atomic
read/modify/write operation, other set misc flags could be lost clearing flags
in that manner.