Get rid of ambiguous OutputType::NONE value

Based on suggestion by Pieter Wuille <pieter.wuille@gmail.com> at
https://github.com/bitcoin/bitcoin/pull/12119#issuecomment-357982763

After #12119, the NONE output type was overloaded to refer to either an output
type that couldn't be parsed, or to an automatic change output mode.  This
change drops the NONE enum and uses a simple bool indicate parse failure, and a
new CHANGE_AUTO enum to refer the change output type.

This change is almost a pure refactoring except it makes RPCs reject empty
string ("") address types instead of treating them like they were unset. This
simplifies the parsing code a little bit and could prevent RPC usage mistakes.
It's noted in the release notes.

doc/release-notes.md

@@ -104,6 +104,11 @@ Low-level RPC changes
   now the empty string `""` instead of `"wallet.dat"`. If bitcoin is started
   with any `-wallet=<path>` options, there is no change in behavior, and the
   name of any wallet is just its `<path>` string.
+- Passing an empty string (`""`) as the `address_type` parameter to
+  `getnewaddress`, `getrawchangeaddress`, `addmultisigaddress`,
+  `fundrawtransaction` RPCs is now an error. Previously, this would fall back
+  to using the default address type. It is still possible to pass null or leave
+  the parameter unset to use the default address type.
 
 ### Logging