Non-nil means dabbrev package should search *all* buffers.
Dabbrev always searches the current buffer first. Then, if
`dabbrev-check-other-buffers' says so, it searches the buffers
designated by `dabbrev-select-buffers-function'.
Then, if `dabbrev-check-all-buffers' is non-nil, dabbrev searches
all the other buffers, except those named in `dabbrev-ignored-buffer-names',
or matched by `dabbrev-ignored-regexps'.