Modify ProcessBlock to accept behavior flags.

This commit change the ProcessBlock function to accept a new type named
BehaviorFlags in place of the current fastAdd parameter.

This has been done to pave the way for adding more control over the checks
that are performed such as avoiding the proof of work checks which will be
in an upcoming commit.  A bitmask was chosen because it will allow the
ProcessBlock API to remain a little more stable since new flag additions
that change the behavior are only likely to be used by new code because
adding flags does not change the existing behavior.

ok @jrick