txscript: mergeMultiSig function def order cleanup.
txscript: mergeMultiSig function def order cleanup.

This moves the function definition for mergeMultiSig so it is more
consistent with the preferred order used through the codebase.  In
particular, the functions are defined before they're first used and
generally as close as possible to the first use when they're defined in
the same file.

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