kernel: kill subprocess before closing pipe to it (#5666)

Otherwise we see issues like this on Linux:

    gap> d := DirectoryCurrent();;
    gap> f := Filename(DirectoriesSystemPrograms(), "rev");;
    gap> s := InputOutputLocalProcess(d,f,[]);;
    gap> Sleep(1);
    gap> CloseStream(s); Print("\n");
    rev: stdin

src/iostream.c

@@ -1010,10 +1010,10 @@ static Obj FuncCLOSE_PTY_IOSTREAM(Obj self, Obj stream)
 
     // Close down the child
     int status;
+    kill(PtyIOStreams[pty].childPID, SIGTERM);
     int retcode = close(PtyIOStreams[pty].ptyFD);
     if (retcode)
         Pr("Strange close return code %d\n", retcode, 0);
-    kill(PtyIOStreams[pty].childPID, SIGTERM);
     // GAP (or another library) might wait on this PID before
     // we handle it. If that happens, waitpid will return -1.
     retcode = waitpid(PtyIOStreams[pty].childPID, &status, WNOHANG);

tst/testspecial/child-process.g

@@ -0,0 +1,5 @@
+d := DirectoryCurrent();;
+f := Filename(DirectoriesSystemPrograms(), "rev");;
+s := InputOutputLocalProcess(d,f,[]);;
+Sleep(1);
+CloseStream(s); Print("\n");

tst/testspecial/child-process.g.out

@@ -0,0 +1,7 @@
+gap> d := DirectoryCurrent();;
+gap> f := Filename(DirectoriesSystemPrograms(), "rev");;
+gap> s := InputOutputLocalProcess(d,f,[]);;
+gap> Sleep(1);
+gap> CloseStream(s); Print("\n");
+
+gap> QUIT;