txscript: Optimize typeOfScript multisig.
txscript: Optimize typeOfScript multisig.

This continues the process of converting the typeOfScript function to
use a combination of raw script analysis and the new tokenizer instead
of the far less efficient parsed opcodes.

In particular, for this commit, since the ability to detect multisig
scripts via the new tokenizer is now available, the function is simply
updated to make use of it.

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