script: match multisigs with up to MAX_PUBKEYS_PER_MULTISIG keys

We were previously ruling out 17-20 pubkeys multisig, while they are
only invalid under P2SH context.
This makes multisigs with up to 20 keys be detected as valid by the
solver. This is however *not* a policy change as it would only apply
to bare multisigs, which are already limited to 3 pubkeys.

Note that this does not change the sigOpCount calculation (as it would
break consensus). Therefore 1-16 keys multisigs are counted as 1-16 sigops
and 17-20 keys multisigs are counted as 20 sigops.

Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

src/script/interpreter.cpp

@@ -225,7 +225,7 @@ bool static CheckPubKeyEncoding(const valtype &vchPubKey, unsigned int flags, co
     return true;
 }
 
-bool static CheckMinimalPush(const valtype& data, opcodetype opcode) {
+bool CheckMinimalPush(const valtype& data, opcodetype opcode) {
     // Excludes OP_1NEGATE, OP_1-16 since they are by definition minimal
     assert(0 <= opcode && opcode <= OP_PUSHDATA4);
     if (data.size() == 0) {