[u104][vim] 1. <s-tab> not working; use <leader><tab> instead> 2. removing <c-[> in VimEnter.

1. <s-tab> just do not work. use <leader><tab> instead.
   <leader><tab> go to previous tab.
   <leader><leader><tab> go to last focused tab.
2. Not found ghost insert mode on u104. Remove it locally.
Merge remote-tracking branch 'origin/u104' into freebsd91p7