partly rewritten and extended.

partly rewritten and extended.
(skeleton-filter, skeleton-untabify, skeleton-further-elements)
(skeleton-abbrev-cleanup): New variables
(skeleton-proxy, skeleton-abbrev-cleanup): New functions
(skeleton-insert): Sublanguage element < must now be handled via
`skeleton-further-elements' (used only in sh-script and ada).  Lisp
expressions can be quoted to ignore the return value.
(skeleton-read): New name for `skeleton-internal-read' because this may
be useful in skeletons.
(local-variables-section): New skeleton command, might go to simple.el.