Fix prefixes names, don't recompute current-buffer.
* helm-dabbrev.el (helm-dabbrev-max-length-result): Default to 20.
(helm-dabbrev-major-mode-assoc): Fix docstring.
(helm-dabbrev--exclude-current-buffer-flag): new internal flag.
(helm-dabbrev--buffer-list): use it.
(helm-dabbrev--get-candidates): don't recompute current-buffer.