patch 8.2.0952: no simple way to interrupt Vim

Problem:    No simple way to interrupt Vim.
Solution:   Add the SigUSR1 autocommand, triggered by SIGUSR1. (Jacob Hayes,
            closes #1718)
diff --git a/src/getchar.c b/src/getchar.c
index fcee7dd..2beffa5 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -2204,6 +2204,13 @@
 	if (has_sound_callback_in_queue())
 	    invoke_sound_callback();
 # endif
+#ifdef SIGUSR1
+	if (got_sigusr1)
+	{
+	    apply_autocmds(EVENT_SIGUSR1, NULL, NULL, FALSE, curbuf);
+	    got_sigusr1 = FALSE;
+	}
+#endif
 	break;
     }