Implement "getchains" RPC command to monitor blockchain forks.

Port over https://github.com/chronokings/huntercoin/pull/19 from
Huntercoin:  This implements a new RPC command "getchains" that can be
used to find all currently active chain heads.  This is similar to the
-printblocktree startup option, but it can be used without restarting
just via the RPC interface on a running daemon.
Rename proposed RPC call to "getchaintips".

Rename the proposed RPC call "getchains" to "getchaintips", as suggested
in the Github pull-request discussion.
Clean up getchaintips RPC call.

Clean up the code for "getchaintips" and extend the help string, in
accordance with discussion in the Github pull request.