Resolve issue 3166.

These changes decode valid SIGHASH types on signatures in assembly (asm) representations of scriptSig scripts.
This squashed commit incorporates substantial helpful feedback from jtimon, laanwj, and sipa.

doc/release-notes.md

@@ -37,6 +37,33 @@ git merge commit are mentioned.
 
 ### RPC and REST
 
+Asm representations of scriptSig signatures now contain SIGHASH type decodes
+----------------------------------------------------------------------------
+
+The `asm` property of each scriptSig now contains the decoded signature hash
+type for each signature that provides a valid defined hash type.
+
+The following items contain assembly representations of scriptSig signatures
+and are affected by this change:
+
+- RPC `getrawtransaction`
+- RPC `decoderawtransaction`
+- REST `/rest/tx/` (JSON format)
+- REST `/rest/block/` (JSON format when including extended tx details)
+- `bitcoin-tx -json`
+
+For example, the `scriptSig.asm` property of a transaction input that
+previously showed an assembly representation of:
+
+    304502207fa7a6d1e0ee81132a269ad84e68d695483745cde8b541e3bf630749894e342a022100c1f7ab20e13e22fb95281a870f3dcf38d782e53023ee313d741ad0cfbc0c509001
+
+now shows as:
+
+    304502207fa7a6d1e0ee81132a269ad84e68d695483745cde8b541e3bf630749894e342a022100c1f7ab20e13e22fb95281a870f3dcf38d782e53023ee313d741ad0cfbc0c5090[ALL]
+
+Note that the output of the RPC `decodescript` did not change because it is
+configured specifically to process scriptPubKey and not scriptSig scripts.
+
 ### Configuration and command-line options
 
 ### Block and transaction handling