Add "sg" method to Tramp

Add "sg" method to Tramp

* doc/misc/tramp.texi (Inline methods): Add "sg" method.
(Customizing Completion): Add function `tramp-parse-etc-group'.

* lisp/net/tramp-sh.el (tramp-methods) <sg>: Add.  (Bug#22329)
(tramp-completion-function-alist-sg): New defconst.
(top): Completion function for "sg" is
`tramp-completion-function-alist-sg'.

* lisp/net/tramp.el (tramp-completion-function-alist): Adapt docstring.
(tramp-parse-etc-group, tramp-parse-etc-group-group): New defuns.