Detect any sufficiently long fork and add a warning.

Such a fork is defined as being at least 7 blocks long and
having a tip which is within 72 blocks of our best block.
Call the -alertnotify script when we see a long or invalid fork.
Better warning/"alert" messages for large-work forks.
Merge pull request #2658 from TheBlueMatt/forkalert

Detect any sufficiently long fork and alert the user just like any other alert