Since the user can control which file patterns are handled by the plugin
(via g:GPGFilePattern), the regular expression checking the buffer name
for GPGEdit{Recipients,Options} may not match. A simpler and more
definitive check is to see whether b:GPGCorrespondingTo is set to
determine if the buffer is an existing recipients/options buffer.
Closesjamessan/vim-gnupg#72
Signed-off-by: James McCoy <jamessan@jamessan.com>