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/vim.h b/src/vim.h
index 99afe29..0204aa3 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -1316,6 +1316,7 @@
     EVENT_SESSIONLOADPOST,	// after loading a session file
     EVENT_SHELLCMDPOST,		// after ":!cmd"
     EVENT_SHELLFILTERPOST,	// after ":1,2!cmd", ":w !cmd", ":r !cmd".
+    EVENT_SIGUSR1,		// after the SIGUSR1 signal
     EVENT_SOURCECMD,		// sourcing a Vim script using command
     EVENT_SOURCEPRE,		// before sourcing a Vim script
     EVENT_SOURCEPOST,		// after sourcing a Vim script