Fix stdin ending up not registered after a Quit

If you press Ctrl-C while GDB is processing breakpoint commands the
TRY/CATCH in inferior_event_handler catches the Quit exception and
prints it, and then if the interpreter was running a foreground
execution command, nothing re-adds stdin back in the event loop,
meaning the debug session ends up busted, because the user can't type
anything...

This was exposed by the new gdb.base/bp-cmds-continue-ctrl-c.exp
testcase added later in the series.

gdb/ChangeLog:
2017-11-16  Pedro Alves  <[email protected]>

	* inf-loop.c (inferior_event_handler): Don't swallow the exception
	if the prompt is blocked.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2017-11-16  Pedro Alves  <[email protected]>
+
+	* inf-loop.c (inferior_event_handler): Don't swallow the exception
+	if the prompt is blocked.
+
 2017-11-16  Pedro Alves  <[email protected]>
 
 	* breakpoint.c (insert_bp_location): Replace bp_err and

gdb/inf-loop.c

@@ -73,7 +73,15 @@ inferior_event_handler (enum inferior_event_type event_type,
 	    }
 	  CATCH (e, RETURN_MASK_ALL)
 	    {
-	      exception_print (gdb_stderr, e);
+	      /* If the user was running a foreground execution
+		 command, then propagate the error so that the prompt
+		 can be reenabled.  Otherwise, the user already has
+		 the prompt and is typing some unrelated command, so
+		 just inform the user and swallow the exception.  */
+	      if (current_ui->prompt_state == PROMPT_BLOCKED)
+		throw_exception (e);
+	      else
+		exception_print (gdb_stderr, e);
 	    }
 	  END_CATCH
 	}