Fix termination signal forwarding

When traced command is terminated by a blocked signal, unblock
that signal to ensure strace termination with the same signal.

* strace.c (main): Unblock the signal raised for strace termination.
* NEWS: Mention this change.

NEWS

@@ -5,6 +5,8 @@ Noteworthy changes in release ?.?? (????-??-??)
   * In interactive mode (-I2), those signals that were blocked at startup
     will remain blocked for the whole period of strace execution.
   * strace no longer resets SIGCHLD handler in tracees to the default action.
+  * When traced command is terminated by a blocked signal, strace unblocks
+    that signal to ensure its own termination with the same signal.
 
 Noteworthy changes in release 4.17 (2017-05-24)
 ===============================================

strace.c

@@ -2539,6 +2539,13 @@ main(int argc, char *argv[])
 		exit_code &= 0xff;
 		signal(exit_code, SIG_DFL);
 		raise(exit_code);
+
+		/* Unblock the signal.  */
+		sigset_t mask;
+		sigemptyset(&mask);
+		sigaddset(&mask, exit_code);
+		sigprocmask(SIG_UNBLOCK, &mask, NULL);
+
 		/* Paranoia - what if this signal is not fatal?
 		   Exit with 128 + signo then.  */
 		exit_code += 128;