Force Vim to re-setup the terminal after running a command

After Vim runs certain commands (e.g., pinentry-curses), the terminal is
not returned to its previous state.  This causes some escape sequences,
like arrow keys, to stop being recognized properly by Vim.

Re-setting &term to itself causes Vim to go through its normal terminal
initialization steps which gets things back in working order.

Closes jamessan/vim-gnupg#17
Signed-off-by: James McCoy <vega.james@gmail.com>