Fix posix.fork1() / os.fork1() to only call PyOS_AfterFork() in the child

process rather than both parent and child.

Does anyone actually use fork1()?  It appears to be a Solaris thing
but if Python is built with pthreads on Solaris, fork1() and fork()
should be the same.

Modules/posixmodule.c

@@ -3597,7 +3597,8 @@ posix_fork1(PyObject *self, PyObject *noargs)
 	pid_t pid = fork1();
 	if (pid == -1)
 		return posix_error();
-	PyOS_AfterFork();
+	if (pid == 0)
+		PyOS_AfterFork();
 	return PyInt_FromLong(pid);
 }
 #endif