fix(sched): kill running actions when meowc is interrupted

Ctrl-C killed meowc and left every compiler it had spawned running, still
holding CPU and still about to write into the build directory. There were no
signal handlers at all.

SIGINT and SIGTERM now cancel the same way a build failure does, and the run
returns cleanly so the cache is still written and the exit status is 130. A
second signal falls through to the default handler, so an unresponsive child
cannot trap the user.

Signalling a single pid was not enough: a compiler driver spawns its own
children, and those were reparented to init and survived. Each action is now
placed in its own process group with setsid, and cancellation signals the group,
which also makes the existing failure path reach grandchildren. Unix.wait is
retried on EINTR so a signal arriving mid-wait is not fatal.

bin/meowc.ml

@@ -205,7 +205,7 @@ let execute p ~names =
         end)
       order;
     Printf.printf "\n  %s\n" (Style.dim (Printf.sprintf "%d actions, nothing run" !n));
-    (b, { Sched.built = 0; cached = 0; failed = 0; aborted = 0 })
+    (b, { Sched.built = 0; cached = 0; failed = 0; aborted = 0; interrupted = false })
   end
   else begin
     let cache = Cache.load (Filename.concat p.tc.builddir ".meowc-cache") in
@@ -223,7 +223,10 @@ let execute p ~names =
 let summarise (r : Sched.result) elapsed =
   if not fl.quiet && not fl.dry then begin
     let text =
-      if r.failed > 0 then
+      if r.interrupted then
+        Style.yellow "interrupted"
+        ^ (if r.aborted > 0 then Style.dim (Printf.sprintf ", %d cancelled" r.aborted) else "")
+      else if r.failed > 0 then
         Style.red (Style.plural r.failed "action" ^ " failed")
         ^ (if r.aborted > 0 then Style.dim (Printf.sprintf ", %d cancelled" r.aborted) else "")
       else if r.built = 0 then Style.dim "nothing to do"
@@ -233,6 +236,7 @@ let summarise (r : Sched.result) elapsed =
       (if r.built + r.failed + r.aborted = 0 then "" else "\n")
       (Style.pad 48 text) (Style.dim (duration elapsed))
   end;
+  if r.interrupted then exit 130;
   if r.failed > 0 then exit 1
 
 let cmd_build names =