txscript: Optimize new engine push only script.
txscript: Optimize new engine push only script.

This modifies the check for whether or not a pay-to-script-hash
signature script is a push only script to make use of the new and more
efficient raw script function.

Also, since the script will have already been checked further above when
the ScriptVerifySigPushOnly flags is set, avoid checking it again in
that case.

decred/dcrd,jrick/dcrd,jrick/dcrd,decred/dcrd