Enhance mode-line percentage offset facility, with "%o" and "%q"

Enhance mode-line percentage offset facility, with "%o" and "%q"

"%o" will display the percentage "travel" of the window through the buffer.
"%q" will display a combination of the percentage offsets of the top and
bottom of the window.  The new user option mode-line-percent-position will
facilitate selecting a setting for this part of the mode line.

* lisp/bindings.el (mode-line-percent-position): New customizable user option.
(mode-line-position): Use mode-line-percent-position in place of "%p", etc.

* src/xdisp.c (decode_mode_spec): Add handlers for "%o" and "%q".

* doc/lispref/modes.texi (Mode Line Variables): Document
mode-line-percent-position.
(%-Constructs): Document %o and %q.

* etc/NEWS: Add an entry for these new facilities.