um: Cleanup SIGTERM handling

Richard reported that some UML processes survive if the UML
main process receives a SIGTERM.
This issue was caused by a wrongly placed signal(SIGTERM, SIG_DFL)
in init_new_thread_signals().
It disabled the UML exit handler accidently for some processes.
The correct solution is to disable the fatal handler for all
UML helper threads/processes.
Such that last_ditch_exit() does not get called multiple times
and all processes can exit due to SIGTERM.

Reported-and-tested-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/drivers/ubd.h

@@ -7,7 +7,6 @@
 #ifndef __UM_UBD_USER_H
 #define __UM_UBD_USER_H
 
-extern void ignore_sigwinch_sig(void);
 extern int start_io_thread(unsigned long sp, int *fds_out);
 extern int io_thread(void *arg);
 extern int kernel_fd;

arch/um/drivers/ubd_kern.c

@@ -1476,7 +1476,8 @@ int io_thread(void *arg)
 	struct io_thread_req *req;
 	int n;
 
-	ignore_sigwinch_sig();
+	os_fix_helper_signals();
+
 	while(1){
 		n = os_read_file(kernel_fd, &req,
 				 sizeof(struct io_thread_req *));

arch/um/drivers/ubd_user.c

@@ -21,11 +21,6 @@
 #include "ubd.h"
 #include <os.h>
 
-void ignore_sigwinch_sig(void)
-{
-	signal(SIGWINCH, SIG_IGN);
-}
-
 int start_io_thread(unsigned long sp, int *fd_out)
 {
 	int pid, fds[2], err;