Add more separators to longlines-mode

Add more separators to longlines-mode

* lisp/obsolete/longlines.el (longlines-breakpoint-chars): New
custom to have multiple breakpoint chars.
(longlines-set-breakpoint): Add a target-column parameter and use
`longlines-breakpoint-chars'.
(longlines-find-break-backward, longlines-find-break-foreward):
Use `longlines-breakpoint-chars'.
(longlines-wrap-line): Do not insert space upon merging, just
remove the soft newline. Fix "space before tab" in indent.
(longlines-merge-lines-p): Use the new target-column parameter to
find out if the next line could be merged with the current one.
(longlines-encode-region): Do not replace a soft newline with a
space, just remove it.
* etc/NEWS: New user option 'longlines-breakpoint-chars'