rpc_allow instead of rpc_allow_remote

[dokterbob]

Currently, it is only possible to enable/disable RPC access for 127.0.0.1 rather than being able to specify exactly which address or addresses have access - while for admin-specific commands this is already possible. I would like to propose instead having rpc_allow with a similar syntax as rpc_admin_allow.

This is related to #170 and #174 but is more of a suggested improvement rather than a bugfix; in a lot of cases we want to open up the RPC interface just to a particular network (ie. DMZ, VPN) for just a particular address or a particular group of addresses. Moreover, this would allow for the OS to handle whether or not a socket can be established (rather than rippled itself as with rpc_allow_remote), potentially increasing security.

[vinniefalco]

Obsolete