RPC: strictly require HTTP URI "/"
RPC: strictly require HTTP URI "/"

Previously, JSON-RPC clients accessed URI "/", and the JSON-RPC server
did not care about the URI at all, and would accept any URI as valid.

Change the JSON-RPC server to require URI "/" for all current accesses.

This changes enables the addition of future interfaces at different
URIs, such as pull request #1982 which demonstrates HTTP REST wallet
download.

Or, a future, breaking change in JSON-RPC interface could be introduced
by serving JSON-RPC calls from new URI "/v2/".

dobbscoin/dobbscoin-source,Infernoman/crowncoin,syscoin/syscoin,domob1812/crowncoin,Earlz/dobbscoin-source,Infernoman/crowncoin,Earlz/dobbscoin-source,domob1812/crowncoin,Crowndev/crowncoin,dobbscoin/dobbscoin-source,Crowndev/crowncoin,syscoin/syscoin,Infernoman/crowncoin,Infernoman/crowncoin,dobbscoin/dobbscoin-source,dobbscoin/dobbscoin-source,domob1812/crowncoin,Crowndev/crowncoin,Infernoman/crowncoin,domob1812/crowncoin,domob1812/crowncoin,syscoin/syscoin,syscoin/syscoin,Crowndev/crowncoin,syscoin/syscoin,domob1812/crowncoin,Crowndev/crowncoin,Earlz/dobbscoin-source,syscoin/syscoin,Earlz/dobbscoin-source,Crowndev/crowncoin,dobbscoin/dobbscoin-source,syscoin/syscoin,Infernoman/crowncoin,syscoin/syscoin,Earlz/dobbscoin-source,dobbscoin/dobbscoin-source,Earlz/dobbscoin-source