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)

src/testdir/test_autocmd.vim

@@ -2509,4 +2509,17 @@ func Test_autocmd_deep_nesting()
   autocmd! BufEnter Xfile
 endfunc
 
+" Tests for SigUSR1 autocmd event, which is only available on posix systems.
+func Test_autocmd_sigusr1()
+  CheckUnix
+
+  let g:sigusr1_passed = 0
+  au SigUSR1 * let g:sigusr1_passed = 1
+  call system('/bin/kill -s usr1 ' . getpid())
+  call WaitForAssert({-> assert_true(g:sigusr1_passed)})
+
+  au! SigUSR1
+  unlet g:sigusr1_passed
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab