fork(): Follow-up fix to 17b2a3cfc.

* Ingo rightly noticed that the defer_signals counter is reinitialized on
  thread's user area creation. Setting the flag THREAD_CREATION_FLAG_DEFER_SIGNALS
  indeed gives the expected behavior, deferring signals until undefer_signals() is
  called in the child thread. Thanks for the review and fix suggestion.
* Added a simple test showing the values of the defer_signals counter after fork().

src/system/kernel/team.cpp

@@ -2185,6 +2185,7 @@ fork_team(void)
 		ThreadCreationAttributes threadCreationAttributes(NULL,
 			parentThread->name, parentThread->priority, NULL, team->id, thread);
 		threadCreationAttributes.forkArgs = forkArgs;
+		threadCreationAttributes.flags |= THREAD_CREATION_FLAG_DEFER_SIGNALS;
 		threadID = thread_create_thread(threadCreationAttributes, false);
 		if (threadID < 0) {
 			status = threadID;

src/tests/system/libroot/posix/Jamfile

@@ -1,5 +1,7 @@
 SubDir HAIKU_TOP src tests system libroot posix ;
 
+UsePrivateHeaders libroot system ;
+
 # filter warnings about strftime()-formats in locale_test
 TARGET_WARNING_C++FLAGS_$(TARGET_PACKAGING_ARCH)
 	on [ FGristFiles locale_test.o ] += -Wno-format ;
@@ -34,6 +36,7 @@ SimpleTest test_time : test_time.c ;
 SimpleTest tst-mktime : tst-mktime.c ;
 SimpleTest <test>truncate : truncate.cpp ;
 SimpleTest init_rld_after_fork_test : init_rld_after_fork_test.cpp ;
+SimpleTest user_thread_fork_test : user_thread_fork_test.cpp ;
 
 # XSI tests
 SimpleTest xsi_msg_queue_test1 : xsi_msg_queue_test1.cpp ;

src/tests/system/libroot/posix/user_thread_fork_test.cpp

@@ -0,0 +1,30 @@
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include <OS.h>
+
+#include "user_thread.h"
+
+int
+main()
+{
+	pid_t child = fork();
+	if (child < 0) {
+		fprintf(stderr, "Error: fork() failed: %s\n", strerror(errno));
+		exit(1);
+	}
+
+	if (child > 0) {
+		// the parent process -- wait for the child to finish
+		status_t result;
+		wait_for_thread(child, &result);
+	}
+
+	struct user_thread *t = get_user_thread();
+	printf("defer_signals: %" B_PRId32 "\n", t->defer_signals);
+
+	return 0;
+}