Improve handoff enable/disable config naming

This avoids the confusing double-negative situation where you have to
turn off the disabling of handoff in order to enable it. Now you just
say e.g. "set handoff.inbound=on" to enable inbound handoff, and "set
handoff.inbound=off" to disable it.
Merge pull request #685 from basho/feature/better-handoff-cfg-naming

Improve handoff enable/disable config naming