king: use c vere's heuristics for finding the serf binary

pkg/hs/urbit-king/TODO.md

@@ -63,12 +63,6 @@ Polish:
       changed too quickly.
 
 
-# Finding the Serf Executable
-
-- [ ] Right now, `urbit-worker` is found by looking it up in the PATH. This
-      is wrong, but what is right?
-
-
 # Take Advantage of New IPC Features
 
 - [ ] Hook up `scry` to drivers.

pkg/hs/urbit-king/lib/Urbit/King/Config.hs

@@ -14,7 +14,7 @@ import qualified Urbit.Vere.Serf as Serf
 data PierConfig = PierConfig
   { _pcPierPath  :: FilePath
   , _pcDryRun    :: Bool
-  , _pcSerfExe   :: Text
+  , _pcSerfExe   :: Maybe Text
   , _pcSerfFlags :: [Serf.Flag]
   } deriving (Show)
 

pkg/hs/urbit-king/lib/Urbit/King/Main.hs

@@ -144,7 +144,7 @@ toPierConfig pierPath o@(CLI.Opts{..}) = PierConfig { .. }
  where
   _pcPierPath  = pierPath
   _pcDryRun    = oDryRun || isJust oDryFrom
-  _pcSerfExe   = fromMaybe "urbit-worker" oSerfExe
+  _pcSerfExe   = oSerfExe
   _pcSerfFlags = toSerfFlags o
 
 toNetworkConfig :: CLI.Opts -> NetworkConfig

pkg/hs/urbit-king/lib/Urbit/Vere/Pier.hs

@@ -122,17 +122,24 @@ runSerf
   -> RAcquire e Serf
 runSerf vSlog pax = do
   env <- ask
-  Serf.withSerf (config env)
+  heuSerf <- heuristicallyFindSerf
+  Serf.withSerf (config env heuSerf)
  where
   slog txt = atomically (readTVar vSlog) >>= (\f -> f txt)
-  config env = Serf.Config
-    { scSerf = env ^. pierConfigL . pcSerfExe . to unpack
+  config env heuSerf = Serf.Config
+    { scSerf = env ^. pierConfigL . pcSerfExe . to (unpack . fromMaybe heuSerf)
     , scPier = pax
     , scFlag = env ^. pierConfigL . pcSerfFlags
     , scSlog = \(pri, tank) -> printTank slog pri tank
     , scStdr = \txt -> slog (txt <> "\r\n")
     , scDead = pure () -- TODO: What can be done?
     }
+  heuristicallyFindSerf = serfPrgm <$> listToMaybe <$> getArgs
+  serfPrgm = \case
+    Just (stripSuffix "urbit"      -> Just pfix) -> pfix <> "urbit-worker"
+    Just (stripSuffix "urbit-king" -> Just pfix) -> pfix <> "urbit-worker"
+    _                                            -> "urbit-worker"
+    
 
 
 -- Boot a new ship. ------------------------------------------------------------