New command goto-line-relative (bug#5042, bug#9917)

New command goto-line-relative (bug#5042, bug#9917)

* lisp/simple.el (goto-line-read-args): New function with code from goto-line.
(goto-line): New arg RELATIVE.  Also use 'widen-automatically' to
leave all lines accessible in the narrowed buffer.
(goto-line-relative): New command.

* lisp/info.el (Info-mode-map): Remap 'goto-line' to 'goto-line-relative'.

* doc/emacs/basic.texi (Moving Point):
* doc/emacs/display.texi (Optional Mode Line): Mention goto-line-relative.