Add whitelistforcerelay to control forced relaying.

Also renames whitelistalwaysrelay.

Nodes relay all transactions from whitelisted peers, this
gets in the way of some useful reasons for whitelisting
peers-- for example, bypassing bandwidth limitations.

The purpose of this forced relaying is for specialized gateway
applications where a node is being used as a P2P connection
filter and multiplexer, but where you don't want it getting
in the way of (re-)broadcast.

This change makes it configurable with whitelistforcerelay.