Improve getwork interaction with regtest mode.

Ordinarily, getwork will return an error if btcd is not connected to any
other peers.  This commit relaxes that requirement when running in
regression test mode since it is useful for development purposes.

While here, also improve check which returns an error from getwork is not
current to exclude the check when the best chain height is zero since the
code never believes it is current when at height 0.