daemon: Add 'built-in-builders' RPC.

* nix/libstore/builtins.cc (builtinBuilderNames): New function.
* nix/libstore/builtins.hh (builtinBuilderNames): New declaration.
* nix/libstore/worker-protocol.hh (PROTOCOL_VERSION): Bump to 0x160.
(WorkerOp)[wopBuiltinBuilders]: New value.
* nix/nix-daemon/nix-daemon.cc (performOp): Handle it.
* guix/store.scm (operation-id)[built-in-builders]: New value.
* guix/store.scm (read-arg): Add 'string-list'.
(built-in-builders): New procedure.
* tests/derivations.scm ("built-in-builders"): New test.

nix/libstore/worker-protocol.hh

@@ -6,7 +6,7 @@ namespace nix {
 #define WORKER_MAGIC_1 0x6e697863
 #define WORKER_MAGIC_2 0x6478696f
 
-#define PROTOCOL_VERSION 0x10f
+#define PROTOCOL_VERSION 0x160
 #define GET_PROTOCOL_MAJOR(x) ((x) & 0xff00)
 #define GET_PROTOCOL_MINOR(x) ((x) & 0x00ff)
 
@@ -43,7 +43,8 @@ typedef enum {
     wopQuerySubstitutablePaths = 32,
     wopQueryValidDerivers = 33,
     wopOptimiseStore = 34,
-    wopVerifyStore = 35
+    wopVerifyStore = 35,
+    wopBuiltinBuilders = 80
 } WorkerOp;