vim: preserve the 'show everything' flag when finding next/prev buffer

When show mode is invoked it could be displaying just the matched messages
or everything.  This flag is passed to NM_search_show_thread().  It is then
stored in a buffer variable, b:nm_show_everything, and used for subsequent
calls to NM_search_show_thread() triggered by <Space>, <C-n> and <C-p>.

Signed-off-by: Bart Trojanowski <bart@jukie.net>