Since buffer numbers are unique in Vim, there's no concern about
collisions between options/recipients buffers for different encrypted
buffers.
This also resolves an issue where opening the options buffer would
trigger the recipients buffer (due to the buffer name matching
g:GPGFilePattern.
Closesjamessan/vim-gnupg#89
Signed-off-by: James McCoy <jamessan@jamessan.com>