Rename rpcuser.py to rpcauth.py

This script creates `rpcauth` entries for bitcoin.conf,
not the deprecated `rpcuser` entry, so this changes the name
of the script to match.

share/rpcauth/README.md

@@ -0,0 +1,10 @@
+RPC Tools
+---------------------
+
+### [RPCAuth](/share/rpcauth) ###
+
+Create login credentials for a JSON-RPC user.
+
+Usage:
+
+    ./rpcauth.py <username>

share/rpcuser/README.md

@@ -1,10 +0,0 @@
-RPC Tools
----------------------
-
-### [RPCUser](/share/rpcuser) ###
-
-Create an RPC user login credential.
-
-Usage:
-
-    ./rpcuser.py <username>