preserve_exit: Ignore SIGHUP

closes #9274
ref #9028

If stdin closed then read_error_exit calls preserve_exit. Handling
SIGHUP during preserve_exit would cause a premature teardown, and
conflicts with e.g. ui_bridge_stop which waits for TUI to teardown.

Vim ignores SIGHUP in its prepare_to_exit and getout_preserve_modified
routines:

    /* Ignore SIGHUP, because a dropped connection causes a read error, which
     * makes Vim exit and then handling SIGHUP causes various reentrance
     * problems. */
    signal(SIGHUP, SIG_IGN);

test/functional/autocmd/termclose_spec.lua

@@ -6,6 +6,7 @@ local clear, command, nvim, nvim_dir =
 local eval, eq, retry =
   helpers.eval, helpers.eq, helpers.retry
 local ok = helpers.ok
+local feed = helpers.feed
 local iswin = helpers.iswin
 
 
@@ -87,5 +88,6 @@ describe('TermClose event', function()
 
     command('3bdelete!')
     retry(nil, nil, function() eq('3', eval('g:abuf')) end)
+    feed('<c-c>:qa!<cr>')
   end)
 end)