dcrutil: Lookup params by addr prefix in chaincfg.
dcrutil: Lookup params by addr prefix in chaincfg.

This modifies the code that looks up the params by a given network
prefix to make use of the new function provided by chaincfg in order to
ensure that it works with all registered networks as the package was
intended.

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