Add rejection of non-null CHECKMULTISIG dummy values

This is a source of transaction mutability as the dummy value was
previously not checked and could be modified to something other than the
usual OP_0 value.
Do not trigger a DoS ban if SCRIPT_VERIFY_NULLDUMMY fails
Increase IsStandard() scriptSig length

Removes the limits on number of pubkeys for P2SH CHECKMULTISIG outputs.
Previously with the 500 byte scriptSig limit there were odd restrictions
where even a 1-of-12 P2SH could be spent in a standard transaction(1),
yet multisig scriptPubKey's requiring more signatures quickly ran out of
scriptSig space.

From a "stuff-data-in-the-blockchain" point of view not much has changed
as with the prior commit now only allowing the dummy value to be null
the newly allowed scriptSig space can only be used for signatures. In
any case, just using more outputs is trivial and doesn't cost much.

1) See 779b519480d8c5346de6e635119c7ee772e97ec872240c45e558f582a37b4b73
   Mined by BTC Guild.
Check redeemScript size does not exceed 520 byte limit

redeemScripts >520bytes can't be spent due to the
MAX_SCRIPT_ELEMENT_SIZE limit; previously the addmultisigaddress and
createmultisig RPC calls would let you violate that limit unknowingly.

Also made the wallet code itself check the redeemScript prior to adding
it to the wallet, which in the (rare) instance that a user has added an
invalid oversized redeemScript to their wallet causes an error on
startup. The affected key isn't added to the wallet; other keys are
unaffected.