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/".

thunderrabbit/clams,nochowderforyou/clams,dooglus/clams,Kefkius/clams,robvanmieghem/clams,nochowderforyou/clams,Kefkius/clams,nochowderforyou/clams,dooglus/clams,Jheguy2/Mercury,robvanmieghem/clams,nochowderforyou/clams,Jheguy2/Mercury,thunderrabbit/clams,Kefkius/clams,dooglus/clams,robvanmieghem/clams,Jheguy2/Mercury,Kefkius/clams,Kefkius/clams,nochowderforyou/clams,dooglus/clams,thunderrabbit/clams,Jheguy2/Mercury,robvanmieghem/clams,robvanmieghem/clams,dooglus/clams,thunderrabbit/clams,thunderrabbit/clams,Jheguy2/Mercury