Add a field `strDebugMessage` which can be passed to DoS or Invalid,
and queried using GetDebugMessage() to add extra troubleshooting
information to the validation state.
ffd75ad Enable CHECKLOCKTIMEVERIFY as a standard script verify flag (Peter Todd)
bc60b2b Replace NOP2 with CHECKLOCKTIMEVERIFY (BIP65) (Peter Todd)
48e9c57 Move LOCKTIME_THRESHOLD to src/script/script.h (Peter Todd)
99088d6 Make CScriptNum() take nMaxNumSize as an argument (Peter Todd)