Return a list of other buffers to search for a possible abbrev.
The current buffer is not included in the list.
This function makes a list of all the buffers returned by `buffer-list',
then discards buffers whose names match `dabbrev-ignored-buffer-names'
or `dabbrev-ignored-buffer-regexps'. It also discards buffers for which
`dabbrev-friend-buffer-function', if it is bound, returns nil when called
with the buffer as argument.
It returns the list of the buffers that are not discarded. (fn)