Add helper utility for acquiring the lock

For running commands alongside the server without interfering with its
operation.

flock.sml

@@ -0,0 +1,27 @@
+(*
+  Acquire the lock that is used by the server to run a command without
+  interfering with the server
+*)
+use "regressionLib.sml";
+open regressionLib
+open Posix.Process
+fun main () =
+  let
+    val args = CommandLine.arguments ()
+    val () = assert (not (List.null args)) ["usage: ./flock cmd args ..."]
+    val fd = acquire_lock ()
+  in
+    case fork() of
+      NONE => execp (List.hd args, args)
+    | SOME pid =>
+      let
+        val (pid',status) = wait ()
+        val () = assert (pid=pid') ["wrong child"]
+        val () = Posix.IO.close fd
+      in
+        case status of
+          W_EXITED => ()
+        | W_EXITSTATUS w => exit w
+        | _ => exit (Word8.fromInt 126)
+      end
+  end