Revisions after code review with gmaxwell on #bitcoin-dev

Reimplemented -whiteconnections to count downwards from -maxconnections, instead of extending it upwards.
If user uses whitelist, but does not give -maxconnections, silently increase -maxconnections default, to avoid unwanted reduction in public access.
If user does not use whitelist, disabling whitelist reservation entirely, same reason.
Reworded warning messages.